diff --git a/src/population.rs b/src/population.rs
index 3085e05..21622b4 100644
--- a/src/population.rs
+++ b/src/population.rs
@@ -1,5 +1,4 @@
-use std::sync::Arc;
-use std::sync::Mutex;
+use std::sync::{Arc, RwLock};
 use crate::prelude::*;
 use std::thread;
 
@@ -14,7 +13,7 @@ pub struct Population {
     pub start_infected_ratio: i32,
     pub start_immune_ratio: i32,
     pub start_dead_ratio: i32,
-    pub humans: Arc<Mutex<Vec<Human>>>,
+    pub humans: Arc<RwLock<Vec<Human>>>,
     pub width: i32,
     pub height: i32,
     pub age: i32,
@@ -39,15 +38,15 @@ impl Population {
 
         let size: usize = (width * height) as usize;
 
-        let the_humans_arc = Arc::new(Mutex::new(vec![
+        let mut the_humans = vec![
             Human {
                 x: 0,
                 y: 0,
                 present_state: State::Normal
             };
             size
-        ]));
-        let the_humans = Arc::clone(&the_humans_arc);
+        ];
+
         for x in 0..width {
             for y in 0..height {
                 let idx = human_idx(x, y, width);
@@ -65,9 +64,10 @@ impl Population {
                 {
                     present_state = State::Dead;
                 }
-                the_humans.lock().unwrap()[idx] = Human{x: x, y: y, present_state: present_state};
+                the_humans[idx] = Human{x: x, y: y, present_state: present_state};
             }
         }
+
         Self {
             start_infected_ratio: start_infected_ratio,
             start_immune_ratio: start_immune_ratio,
@@ -76,7 +76,7 @@ impl Population {
             height: height,
             plague: plague,
             age: 0,
-            humans: the_humans_arc,
+            humans: Arc::new(RwLock::new(the_humans)),
             size: size,
         }
     }
@@ -93,10 +93,10 @@ impl Population {
     }
 
     fn is_inside_and_infected(&self, point: Point) -> bool {
-        let the_humans_arc = Arc::clone(&self.humans);
+        let humans = Arc::clone(&self.humans);
         if self.is_inside(&point) {
             let idx = human_idx(point.x, point.y, self.width);
-            let humans = the_humans_arc.lock().unwrap();
+            let humans = humans.read().unwrap();
             if humans[idx].present_state == State::Infected {
                 roll(self.plague.infection_rate)
             } else {
@@ -121,25 +121,29 @@ impl Population {
         let mut stats: [i32; 4] = [0, 0, 0, 0];
         // stats[0] Normal stats[1] Infected stats[2] Immune stats[3] Dead
 
-        let humans = Arc::clone(&self.humans);
-        for h in humans.lock().unwrap().iter() {
-            match h.present_state {
-                State::Normal => {
-                    possible_infected.push(Point{ x: h.x, y: h.y});
-                    stats[0] += 1;
-                }
-                State::Infected => {
-                    people_to_check.push(Point { x: h.x, y: h.y });
-                    stats[1] += 1;
-                }
-                State::Immune => {
-                    stats[2] += 1;
-                }
-                State::Dead => {
-                    stats[3] += 1;
+        {
+            let humans = Arc::clone(&self.humans);
+            let humans = humans.read().unwrap();
+            for h in humans.iter() {
+                match h.present_state {
+                    State::Normal => {
+                        possible_infected.push(Point{ x: h.x, y: h.y});
+                        stats[0] += 1;
+                    }
+                    State::Infected => {
+                        people_to_check.push(Point { x: h.x, y: h.y });
+                        stats[1] += 1;
+                    }
+                    State::Immune => {
+                        stats[2] += 1;
+                    }
+                    State::Dead => {
+                        stats[3] += 1;
+                    }
                 }
             }
         }
+
         //        for pos in &people_to_check {
         for pos in people_to_check.iter() {
             //people_to_check.iter().map(|pos|{
@@ -157,6 +161,7 @@ impl Population {
             }
         }
         for pos in possible_infected.iter() {
+            // infect human if someone near is infected
             let infected: bool = self.is_inside_and_infected(
                 Point {
                     x: pos.x - 1,
@@ -210,18 +215,6 @@ impl Population {
             }
         }
 
-//        for infected_position in people_to_infect.iter() {
-//            // println!("To infect: {:?}", infected_position);
-//            //people_to_infect.iter().map(|infected_position|{
-//            let infected_index = human_idx(infected_position.x, infected_position.y, self.width);
-//            // let _ = infected_position.x;
-//            //DEBUG
-//            //println!("x: {} y: {} index: {}",infected_position.x,infected_position.y,infected_index);
-//            self.humans[infected_index].present_state = State::Infected;
-//            //DEBUG
-//            //println!("Infected someone");
-//        }
-
         let mut threads = vec![];
         {
             let humans = Arc::clone(&self.humans);
@@ -229,7 +222,11 @@ impl Population {
             threads.push(thread::spawn(move || {
                 for infected_position in people_to_infect.iter() {
                     let infected_index = human_idx(infected_position.x, infected_position.y, width);
-                    humans.lock().unwrap()[infected_index].present_state = State::Infected;
+                    {
+                        println!("infect");
+                        let mut humans = humans.write().unwrap();
+                        humans[infected_index].present_state = State::Infected;
+                    }
                 }
             }));
         }
@@ -241,10 +238,14 @@ impl Population {
                 for cured_position in people_to_cure.iter() {
                     //people_to_cure.iter().map(|cured_position|{
                     let cured_index = human_idx(cured_position.x, cured_position.y, width);
-                    if humans.lock().unwrap()[cured_index].present_state != State::Infected {
-                        println!("not infected");
-                    } else {
-                        humans.lock().unwrap()[cured_index].present_state = State::Immune;
+                    {
+                        let humans = humans.read().unwrap();
+                        debug_assert!(humans[cured_index].present_state != State::Infected, "This human should not be infected: {:?}", humans[cured_index].present_state);
+                    }
+                    {
+                        println!("cure");
+                        let mut humans = humans.write().unwrap();
+                        humans[cured_index].present_state = State::Immune;
                     }
                     //DEBUG
                     //println!("Cured someone");
@@ -252,22 +253,31 @@ impl Population {
             }));
         }
 
+        {
+            let humans = Arc::clone(&self.humans);
+            let width = self.width;
+            threads.push(thread::spawn(move || {
+                for dead_position in people_to_kill.iter() {
+                    //people_to_kill.iter().map(|dead_position|{
+                    let dead_index = human_idx(dead_position.x, dead_position.y, width);
+                    {
+                        let humans = humans.read().unwrap();
+                        debug_assert_eq!(humans[dead_index].present_state, State::Dead);
+                    }
+                    {
+                        println!("kill");
+                        let mut humans = humans.write().unwrap();
+                        humans[dead_index].present_state = State::Dead;
+                    }
+                }
+            }));
+        }
+
         for t in threads {
             t.join().unwrap();
         }
 
-        for dead_position in people_to_kill.iter() {
-            let humans = Arc::clone(&self.humans);
-            //people_to_kill.iter().map(|dead_position|{
-            let dead_index = human_idx(dead_position.x, dead_position.y, self.width);
-            if humans.lock().unwrap()[dead_index].present_state == State::Dead {
-                // println!("Already dead");
-            } else {
-                humans.lock().unwrap()[dead_index].present_state = State::Dead;
-            }
-            //DEBUG
-        }
-        assert_eq!(
+        debug_assert_eq!(
             stats[0] + stats[1] + stats[2] + stats[3],
             self.size as i32
         );
@@ -301,452 +311,3 @@ pub fn roll(probability: i32) -> bool {
         false
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-    use parameterized::parameterized;
-
-    #[derive(Debug)]
-    struct Stats {
-        normal: i32,
-        infected: i32,
-        immune: i32,
-        dead: i32,
-    }
-
-    impl Stats {
-        fn new() -> Stats {
-            Stats {
-                normal: 0,
-                infected: 0,
-                immune: 0,
-                dead: 0,
-            }
-        }
-    }
-
-    fn humans_stats(humans: &Vec<Human>) -> Stats {
-        let mut stats: Stats = Stats::new();
-        for human in humans.iter() {
-            match human.present_state {
-                State::Normal => {
-                    stats.normal += 1;
-                }
-                State::Infected => {
-                    stats.infected += 1;
-                }
-                State::Immune => {
-                    stats.immune += 1;
-                }
-                State::Dead => {
-                    stats.dead += 1;
-                }
-            }
-        }
-        stats
-    }
-
-    #[parameterized(x = {
-        2, 3, 5
-    }, y = {
-        1, 4, 0
-    }, width = {
-        3, 5, 7
-    }, res = {
-        5, 23, 5
-    })]
-    fn test_human_idx(x: i32, y: i32, width: i32, res: usize) {
-        assert_eq!(human_idx(x, y, width), res);
-    }
-
-    #[test]
-    fn test_human_stats() {
-        let mut humans: Vec<Human> = Vec::with_capacity(10);
-        let mut stats: Stats;
-
-        for _ in 0..10 {
-            humans.push(Human {
-                present_state: State::Normal,
-                x: 0,
-                y: 0,
-            });
-        }
-        stats = humans_stats(&humans);
-        assert_eq!(stats.normal, 10);
-
-        for x in 0..2 {
-            humans[x].present_state = State::Infected;
-        }
-        for x in 2..5 {
-            humans[x].present_state = State::Immune;
-        }
-        for x in 5..9 {
-            humans[x].present_state = State::Dead;
-        }
-        stats = humans_stats(&humans);
-        assert_eq!(stats.normal, 1);
-        assert_eq!(stats.infected, 2);
-        assert_eq!(stats.immune, 3);
-        assert_eq!(stats.dead, 4);
-    }
-
-    #[test]
-    fn population_new() {
-        let disease = Disease::new(20, 10, 5, String::from("Covid 44"));
-        let (width, height) = (5, 7);
-        let population = Population::new(20, 10, 5, 5, 7, disease);
-        let humans = Arc::clone(&population.humans);
-        assert_eq!(humans.lock().unwrap().len(), 5 * 7);
-        for h in humans.lock().unwrap().iter() {
-            let idx = human_idx(h.x, h.y, width);
-            assert_eq!(humans.lock().unwrap()[idx].x, h.x, "coordinates should match");
-            assert_eq!(humans.lock().unwrap()[idx].y, h.y, "coordinates should match");
-        }
-        assert_eq!(humans.lock().unwrap().len(), (width * height) as usize);
-    }
-
-    #[test]
-    fn population_gen() {
-        let disease = Disease::new(20, 10, 5, String::from("Covid 44"));
-        let (width, height) = (5, 7);
-        let population = Population::new(20, 10, 5, 5, 7, disease);
-
-        let stats: Stats = humans_stats(&population.humans);
-        println!("Stats: {:?}", stats);
-
-        assert_eq!(
-            stats.normal + stats.infected + stats.immune + stats.dead,
-            width * height
-        );
-    }
-
-    #[test]
-    fn plague_init_stats() {
-        let mut disease: Disease;
-        let mut population: Population;
-        let mut stats: Stats;
-        let (width, height) = (5, 7);
-
-        disease = Disease::new(0, 0, 0, String::from("Test"));
-        population = Population::new(0, 0, 0, width, height, disease);
-        stats = humans_stats(&population.humans);
-        println!("should be normal: {:?}", stats);
-        assert_eq!(stats.normal, width * height);
-
-        disease = Disease::new(0, 0, 0, String::from("Test"));
-        population = Population::new(100, 0, 0, width, height, disease);
-        stats = humans_stats(&population.humans);
-        println!("should be infected: {:?}", stats);
-        assert_eq!(stats.infected, width * height);
-
-        disease = Disease::new(0, 0, 0, String::from("Test"));
-        population = Population::new(0, 100, 0, width, height, disease);
-        stats = humans_stats(&population.humans);
-        println!("should be immune: {:?}", stats);
-        assert_eq!(stats.immune, width * height);
-
-        disease = Disease::new(0, 0, 0, String::from("Test"));
-        population = Population::new(0, 0, 100, width, height, disease);
-        stats = humans_stats(&population.humans);
-        println!("should be dead: {:?}", stats);
-        assert_eq!(stats.dead, width * height);
-    }
-
-    #[parameterized(rate = {0, 100}, expected = {false, true})]
-    fn roll_test(rate: i32, expected: bool) {
-        let tries = 100000;
-        let mut result = 0;
-        println!("Testing roll, rate {}, expected {}", rate, expected);
-        for _x in 0..tries {
-            if roll(rate) == expected {
-                result += 1;
-            }
-        }
-        assert_eq!(result, tries);
-    }
-
-    #[test]
-    fn propagate_simple() {
-        let disease: Disease = Disease::new(0, 0, 100, String::from("Deadly"));
-        let mut population: Population = Population::new(100, 0, 0, 10, 10, disease);
-        let mut stats: Stats;
-        let mut propagate_stats: [i32; 4];
-
-        // infect every one
-        stats = humans_stats(&population.humans);
-        println!("stats after init: {:?}", stats);
-        assert_eq!(stats.infected, 100, "everybody should be infected");
-
-        // kill every one
-        propagate_stats = population.propagate();
-        stats = humans_stats(&population.humans);
-        println!("propate_stats: {:?}", propagate_stats);
-        assert_eq!(propagate_stats, [0, 100, 0, 0]);
-        assert_eq!(stats.normal, 0);
-        assert_eq!(stats.infected, 0);
-        assert_eq!(stats.immune, 0);
-        assert_eq!(stats.dead, 100);
-
-        for _x in 0..100 {
-            propagate_stats = population.propagate();
-            stats = humans_stats(&population.humans);
-            println!("propate_stats: {:?}", propagate_stats);
-            assert_eq!(propagate_stats, [0, 0, 0, 100]);
-            assert_eq!(stats.normal, 0);
-            assert_eq!(stats.infected, 0);
-            assert_eq!(stats.immune, 0);
-            assert_eq!(stats.dead, 100);
-        }
-    }
-
-    #[test]
-    fn propagate_infect_all() {
-        let disease: Disease = Disease::new(100, 0, 0, String::from("Deadly"));
-        let mut population: Population = Population::new(0, 0, 0, 3, 3, disease);
-        let mut stats: Stats;
-        let mut propagate_stats: [i32; 4];
-
-        // start with normal population
-        population.humans = vec![
-            Human {
-                present_state: State::Normal,
-                x: 0,
-                y: 0,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 1,
-                y: 0,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 2,
-                y: 0,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 0,
-                y: 1,
-            },
-            Human {
-                present_state: State::Infected,
-                x: 1,
-                y: 1,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 2,
-                y: 1,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 0,
-                y: 2,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 1,
-                y: 2,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 2,
-                y: 2,
-            },
-        ];
-        stats = humans_stats(&population.humans);
-        println!("stats after init: {:?}", stats);
-        assert_eq!(stats.normal, 8);
-
-        // kill every one
-        propagate_stats = population.propagate();
-        stats = humans_stats(&population.humans);
-        println!("propate_stats: {:?}", propagate_stats);
-        assert_eq!(propagate_stats, [8, 1, 0, 0]);
-        assert_eq!(stats.normal, 0);
-        assert_eq!(stats.infected, 9);
-        assert_eq!(stats.immune, 0);
-        assert_eq!(stats.dead, 0);
-
-        for _x in 0..100 {
-            propagate_stats = population.propagate();
-            stats = humans_stats(&population.humans);
-            println!("propate_stats: {:?}", propagate_stats);
-            assert_eq!(propagate_stats, [0, 9, 0, 0]);
-            assert_eq!(stats.normal, 0);
-            assert_eq!(stats.infected, 9);
-            assert_eq!(stats.immune, 0);
-            assert_eq!(stats.dead, 0);
-        }
-    }
-
-    #[test]
-    fn propagate_infect_cure_all() {
-        let disease: Disease = Disease::new(100, 100, 0, String::from("Deadly"));
-        let mut population: Population = Population::new(0, 0, 0, 3, 3, disease);
-        let mut stats: Stats;
-        let mut propagate_stats: [i32; 4];
-
-        // start with normal population
-        population.humans = vec![
-            Human {
-                present_state: State::Normal,
-                x: 0,
-                y: 0,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 1,
-                y: 0,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 2,
-                y: 0,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 0,
-                y: 1,
-            },
-            Human {
-                present_state: State::Infected,
-                x: 1,
-                y: 1,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 2,
-                y: 1,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 0,
-                y: 2,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 1,
-                y: 2,
-            },
-            Human {
-                present_state: State::Normal,
-                x: 2,
-                y: 2,
-            },
-        ];
-        stats = humans_stats(&population.humans);
-        println!("stats after init: {:?}", stats);
-        assert_eq!(stats.normal, 8);
-
-        // infect every one
-        propagate_stats = population.propagate();
-        stats = humans_stats(&population.humans);
-        println!("propate_stats: {:?}", propagate_stats);
-        println!("population: {:?}", stats);
-        assert_eq!(propagate_stats, [8, 1, 0, 0]);
-        assert_eq!(stats.normal, 0);
-        assert_eq!(stats.infected, 8);
-        assert_eq!(stats.immune, 1);
-        assert_eq!(stats.dead, 0);
-
-        // cure every one
-        propagate_stats = population.propagate();
-        stats = humans_stats(&population.humans);
-        println!("propate_stats: {:?}", propagate_stats);
-        println!("population: {:?}", stats);
-        assert_eq!(propagate_stats, [0, 8, 1, 0]);
-        assert_eq!(stats.normal, 0);
-        assert_eq!(stats.infected, 0);
-        assert_eq!(stats.immune, 9);
-        assert_eq!(stats.dead, 0);
-
-        // then
-        for _x in 0..100 {
-            propagate_stats = population.propagate();
-            stats = humans_stats(&population.humans);
-            println!("propate_stats: {:?}", propagate_stats);
-            println!("population: {:?}", stats);
-            assert_eq!(propagate_stats, [0, 0, 9, 0]);
-            assert_eq!(stats.normal, 0);
-            assert_eq!(stats.infected, 0);
-            assert_eq!(stats.immune, 9);
-            assert_eq!(stats.dead, 0);
-        }
-    }
-
-    #[parameterized(infection_start = {0, 50, 100})]
-    fn propagate_harmless(infection_start: i32) {
-        let disease: Disease = Disease::new(0, 0, 0, String::from("Harmless"));
-        let (width, height) = (100, 100);
-        let mut population: Population = Population::new(infection_start, 0, 0, width, height, disease);
-        let stats_before: Stats;
-        let stats_after: Stats;
-
-        stats_before = humans_stats(&population.humans);
-        let should_be_infected = population.size as i32 * infection_start / 100;
-        let tolerance = (should_be_infected as f32 * 0.20) as i32;
-        println!("{:?}", stats_before);
-        assert!(stats_before.infected <= should_be_infected + tolerance, "{} infected, should be less than {}", stats_before.infected, should_be_infected + tolerance);
-        assert!(stats_before.infected >= should_be_infected - tolerance, "{} infected, should be more than {}", stats_before.infected, should_be_infected - tolerance);
-
-        population.propagate();
-
-        stats_after = humans_stats(&population.humans);
-        assert_eq!(stats_before.infected, stats_after.infected, "no one should have been infected");
-    }
-
-    #[parameterized(infection_rate = {0, 100, 0}, death_rate = {0, 0, 100}, infected_expected = {0, 1, 0})]
-    fn propagate_test(infection_rate: i32, death_rate: i32, infected_expected: i32) {
-        let disease: Disease;
-        let mut population: Population;
-        let mut stats: Stats;
-        let (width, height) = (100, 100);
-        let start_infected = 50;
-
-        println!(
-            "infection rate: {}, death_rate: {}",
-            infection_rate, death_rate
-        );
-
-        disease = Disease::new(infection_rate, 0, death_rate, String::from("Test"));
-        population = Population::new(start_infected, 0, 0, width, height, disease);
-
-        stats = humans_stats(&population.humans);
-        println!("Population after init: {:?}", stats);
-
-        // total * proba - 20% < infected < total * proba + 20%
-        let infected_at_start_proba = width * height * start_infected / 100;
-        let infected_tolerance = ((width * height) as f32 * 0.2) as i32;
-        assert!(stats.infected <= infected_at_start_proba + infected_tolerance);
-        assert!(stats.infected >= infected_at_start_proba - infected_tolerance);
-        assert_eq!(stats.dead, 0);
-
-        let infected_at_start = stats.infected;
-        let dead_at_start = stats.dead;
-
-        let propa_stats: [i32; 4] = population.propagate();
-
-        assert!(propa_stats[3] >= dead_at_start);
-
-        if death_rate == 0 {
-            assert_eq!(propa_stats[3], 0, "no human should have died");
-        }
-
-        stats = humans_stats(&population.humans);
-        println!("Population after propagate: {:?}", stats);
-
-        assert!(stats.infected <= infected_at_start + width * height * infected_expected);
-
-        let should_be_dead = infected_at_start * death_rate / 100;
-        let dead_tolerance = (should_be_dead as f32 * 0.20) as i32;
-
-        assert!(
-            stats.dead <= should_be_dead + dead_tolerance,
-            "death count should be less or equal than {}",
-            should_be_dead + dead_tolerance
-        );
-        assert!(stats.dead >= should_be_dead - dead_tolerance);
-    }
-}
diff --git a/src/tests.rs b/src/tests.rs
new file mode 100644
index 0000000..0b3e5c4
--- /dev/null
+++ b/src/tests.rs
@@ -0,0 +1,448 @@
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use parameterized::parameterized;
+
+    #[derive(Debug)]
+    struct Stats {
+        normal: i32,
+        infected: i32,
+        immune: i32,
+        dead: i32,
+    }
+
+    impl Stats {
+        fn new() -> Stats {
+            Stats {
+                normal: 0,
+                infected: 0,
+                immune: 0,
+                dead: 0,
+            }
+        }
+    }
+
+    fn humans_stats(humans: &Vec<Human>) -> Stats {
+        let mut stats: Stats = Stats::new();
+        for human in humans.iter() {
+            match human.present_state {
+                State::Normal => {
+                    stats.normal += 1;
+                }
+                State::Infected => {
+                    stats.infected += 1;
+                }
+                State::Immune => {
+                    stats.immune += 1;
+                }
+                State::Dead => {
+                    stats.dead += 1;
+                }
+            }
+        }
+        stats
+    }
+
+    #[parameterized(x = {
+        2, 3, 5
+    }, y = {
+        1, 4, 0
+    }, width = {
+        3, 5, 7
+    }, res = {
+        5, 23, 5
+    })]
+    fn test_human_idx(x: i32, y: i32, width: i32, res: usize) {
+        assert_eq!(human_idx(x, y, width), res);
+    }
+
+    #[test]
+    fn test_human_stats() {
+        let mut humans: Vec<Human> = Vec::with_capacity(10);
+        let mut stats: Stats;
+
+        for _ in 0..10 {
+            humans.push(Human {
+                present_state: State::Normal,
+                x: 0,
+                y: 0,
+            });
+        }
+        stats = humans_stats(&humans);
+        assert_eq!(stats.normal, 10);
+
+        for x in 0..2 {
+            humans[x].present_state = State::Infected;
+        }
+        for x in 2..5 {
+            humans[x].present_state = State::Immune;
+        }
+        for x in 5..9 {
+            humans[x].present_state = State::Dead;
+        }
+        stats = humans_stats(&humans);
+        assert_eq!(stats.normal, 1);
+        assert_eq!(stats.infected, 2);
+        assert_eq!(stats.immune, 3);
+        assert_eq!(stats.dead, 4);
+    }
+
+    #[test]
+    fn population_new() {
+        let disease = Disease::new(20, 10, 5, String::from("Covid 44"));
+        let (width, height) = (5, 7);
+        let population = Population::new(20, 10, 5, 5, 7, disease);
+        let humans = Arc::clone(&population.humans);
+        assert_eq!(humans.lock().unwrap().len(), 5 * 7);
+        for h in humans.lock().unwrap().iter() {
+            let idx = human_idx(h.x, h.y, width);
+            assert_eq!(humans.lock().unwrap()[idx].x, h.x, "coordinates should match");
+            assert_eq!(humans.lock().unwrap()[idx].y, h.y, "coordinates should match");
+        }
+        assert_eq!(humans.lock().unwrap().len(), (width * height) as usize);
+    }
+
+    #[test]
+    fn population_gen() {
+        let disease = Disease::new(20, 10, 5, String::from("Covid 44"));
+        let (width, height) = (5, 7);
+        let population = Population::new(20, 10, 5, 5, 7, disease);
+
+        let stats: Stats = humans_stats(&population.humans);
+        println!("Stats: {:?}", stats);
+
+        assert_eq!(
+            stats.normal + stats.infected + stats.immune + stats.dead,
+            width * height
+        );
+    }
+
+    #[test]
+    fn plague_init_stats() {
+        let mut disease: Disease;
+        let mut population: Population;
+        let mut stats: Stats;
+        let (width, height) = (5, 7);
+
+        disease = Disease::new(0, 0, 0, String::from("Test"));
+        population = Population::new(0, 0, 0, width, height, disease);
+        stats = humans_stats(&population.humans);
+        println!("should be normal: {:?}", stats);
+        assert_eq!(stats.normal, width * height);
+
+        disease = Disease::new(0, 0, 0, String::from("Test"));
+        population = Population::new(100, 0, 0, width, height, disease);
+        stats = humans_stats(&population.humans);
+        println!("should be infected: {:?}", stats);
+        assert_eq!(stats.infected, width * height);
+
+        disease = Disease::new(0, 0, 0, String::from("Test"));
+        population = Population::new(0, 100, 0, width, height, disease);
+        stats = humans_stats(&population.humans);
+        println!("should be immune: {:?}", stats);
+        assert_eq!(stats.immune, width * height);
+
+        disease = Disease::new(0, 0, 0, String::from("Test"));
+        population = Population::new(0, 0, 100, width, height, disease);
+        stats = humans_stats(&population.humans);
+        println!("should be dead: {:?}", stats);
+        assert_eq!(stats.dead, width * height);
+    }
+
+    #[parameterized(rate = {0, 100}, expected = {false, true})]
+    fn roll_test(rate: i32, expected: bool) {
+        let tries = 100000;
+        let mut result = 0;
+        println!("Testing roll, rate {}, expected {}", rate, expected);
+        for _x in 0..tries {
+            if roll(rate) == expected {
+                result += 1;
+            }
+        }
+        assert_eq!(result, tries);
+    }
+
+    #[test]
+    fn propagate_simple() {
+        let disease: Disease = Disease::new(0, 0, 100, String::from("Deadly"));
+        let mut population: Population = Population::new(100, 0, 0, 10, 10, disease);
+        let mut stats: Stats;
+        let mut propagate_stats: [i32; 4];
+
+        // infect every one
+        stats = humans_stats(&population.humans);
+        println!("stats after init: {:?}", stats);
+        assert_eq!(stats.infected, 100, "everybody should be infected");
+
+        // kill every one
+        propagate_stats = population.propagate();
+        stats = humans_stats(&population.humans);
+        println!("propate_stats: {:?}", propagate_stats);
+        assert_eq!(propagate_stats, [0, 100, 0, 0]);
+        assert_eq!(stats.normal, 0);
+        assert_eq!(stats.infected, 0);
+        assert_eq!(stats.immune, 0);
+        assert_eq!(stats.dead, 100);
+
+        for _x in 0..100 {
+            propagate_stats = population.propagate();
+            stats = humans_stats(&population.humans);
+            println!("propate_stats: {:?}", propagate_stats);
+            assert_eq!(propagate_stats, [0, 0, 0, 100]);
+            assert_eq!(stats.normal, 0);
+            assert_eq!(stats.infected, 0);
+            assert_eq!(stats.immune, 0);
+            assert_eq!(stats.dead, 100);
+        }
+    }
+
+    #[test]
+    fn propagate_infect_all() {
+        let disease: Disease = Disease::new(100, 0, 0, String::from("Deadly"));
+        let mut population: Population = Population::new(0, 0, 0, 3, 3, disease);
+        let mut stats: Stats;
+        let mut propagate_stats: [i32; 4];
+
+        // start with normal population
+        population.humans = vec![
+            Human {
+                present_state: State::Normal,
+                x: 0,
+                y: 0,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 1,
+                y: 0,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 2,
+                y: 0,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 0,
+                y: 1,
+            },
+            Human {
+                present_state: State::Infected,
+                x: 1,
+                y: 1,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 2,
+                y: 1,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 0,
+                y: 2,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 1,
+                y: 2,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 2,
+                y: 2,
+            },
+        ];
+        stats = humans_stats(&population.humans);
+        println!("stats after init: {:?}", stats);
+        assert_eq!(stats.normal, 8);
+
+        // kill every one
+        propagate_stats = population.propagate();
+        stats = humans_stats(&population.humans);
+        println!("propate_stats: {:?}", propagate_stats);
+        assert_eq!(propagate_stats, [8, 1, 0, 0]);
+        assert_eq!(stats.normal, 0);
+        assert_eq!(stats.infected, 9);
+        assert_eq!(stats.immune, 0);
+        assert_eq!(stats.dead, 0);
+
+        for _x in 0..100 {
+            propagate_stats = population.propagate();
+            stats = humans_stats(&population.humans);
+            println!("propate_stats: {:?}", propagate_stats);
+            assert_eq!(propagate_stats, [0, 9, 0, 0]);
+            assert_eq!(stats.normal, 0);
+            assert_eq!(stats.infected, 9);
+            assert_eq!(stats.immune, 0);
+            assert_eq!(stats.dead, 0);
+        }
+    }
+
+    #[test]
+    fn propagate_infect_cure_all() {
+        let disease: Disease = Disease::new(100, 100, 0, String::from("Deadly"));
+        let mut population: Population = Population::new(0, 0, 0, 3, 3, disease);
+        let mut stats: Stats;
+        let mut propagate_stats: [i32; 4];
+
+        // start with normal population
+        population.humans = vec![
+            Human {
+                present_state: State::Normal,
+                x: 0,
+                y: 0,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 1,
+                y: 0,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 2,
+                y: 0,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 0,
+                y: 1,
+            },
+            Human {
+                present_state: State::Infected,
+                x: 1,
+                y: 1,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 2,
+                y: 1,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 0,
+                y: 2,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 1,
+                y: 2,
+            },
+            Human {
+                present_state: State::Normal,
+                x: 2,
+                y: 2,
+            },
+        ];
+        stats = humans_stats(&population.humans);
+        println!("stats after init: {:?}", stats);
+        assert_eq!(stats.normal, 8);
+
+        // infect every one
+        propagate_stats = population.propagate();
+        stats = humans_stats(&population.humans);
+        println!("propate_stats: {:?}", propagate_stats);
+        println!("population: {:?}", stats);
+        assert_eq!(propagate_stats, [8, 1, 0, 0]);
+        assert_eq!(stats.normal, 0);
+        assert_eq!(stats.infected, 8);
+        assert_eq!(stats.immune, 1);
+        assert_eq!(stats.dead, 0);
+
+        // cure every one
+        propagate_stats = population.propagate();
+        stats = humans_stats(&population.humans);
+        println!("propate_stats: {:?}", propagate_stats);
+        println!("population: {:?}", stats);
+        assert_eq!(propagate_stats, [0, 8, 1, 0]);
+        assert_eq!(stats.normal, 0);
+        assert_eq!(stats.infected, 0);
+        assert_eq!(stats.immune, 9);
+        assert_eq!(stats.dead, 0);
+
+        // then
+        for _x in 0..100 {
+            propagate_stats = population.propagate();
+            stats = humans_stats(&population.humans);
+            println!("propate_stats: {:?}", propagate_stats);
+            println!("population: {:?}", stats);
+            assert_eq!(propagate_stats, [0, 0, 9, 0]);
+            assert_eq!(stats.normal, 0);
+            assert_eq!(stats.infected, 0);
+            assert_eq!(stats.immune, 9);
+            assert_eq!(stats.dead, 0);
+        }
+    }
+
+    #[parameterized(infection_start = {0, 50, 100})]
+    fn propagate_harmless(infection_start: i32) {
+        let disease: Disease = Disease::new(0, 0, 0, String::from("Harmless"));
+        let (width, height) = (100, 100);
+        let mut population: Population = Population::new(infection_start, 0, 0, width, height, disease);
+        let stats_before: Stats;
+        let stats_after: Stats;
+
+        stats_before = humans_stats(&population.humans);
+        let should_be_infected = population.size as i32 * infection_start / 100;
+        let tolerance = (should_be_infected as f32 * 0.20) as i32;
+        println!("{:?}", stats_before);
+        assert!(stats_before.infected <= should_be_infected + tolerance, "{} infected, should be less than {}", stats_before.infected, should_be_infected + tolerance);
+        assert!(stats_before.infected >= should_be_infected - tolerance, "{} infected, should be more than {}", stats_before.infected, should_be_infected - tolerance);
+
+        population.propagate();
+
+        stats_after = humans_stats(&population.humans);
+        assert_eq!(stats_before.infected, stats_after.infected, "no one should have been infected");
+    }
+
+    #[parameterized(infection_rate = {0, 100, 0}, death_rate = {0, 0, 100}, infected_expected = {0, 1, 0})]
+    fn propagate_test(infection_rate: i32, death_rate: i32, infected_expected: i32) {
+        let disease: Disease;
+        let mut population: Population;
+        let mut stats: Stats;
+        let (width, height) = (100, 100);
+        let start_infected = 50;
+
+        println!(
+            "infection rate: {}, death_rate: {}",
+            infection_rate, death_rate
+        );
+
+        disease = Disease::new(infection_rate, 0, death_rate, String::from("Test"));
+        population = Population::new(start_infected, 0, 0, width, height, disease);
+
+        stats = humans_stats(&population.humans);
+        println!("Population after init: {:?}", stats);
+
+        // total * proba - 20% < infected < total * proba + 20%
+        let infected_at_start_proba = width * height * start_infected / 100;
+        let infected_tolerance = ((width * height) as f32 * 0.2) as i32;
+        assert!(stats.infected <= infected_at_start_proba + infected_tolerance);
+        assert!(stats.infected >= infected_at_start_proba - infected_tolerance);
+        assert_eq!(stats.dead, 0);
+
+        let infected_at_start = stats.infected;
+        let dead_at_start = stats.dead;
+
+        let propa_stats: [i32; 4] = population.propagate();
+
+        assert!(propa_stats[3] >= dead_at_start);
+
+        if death_rate == 0 {
+            assert_eq!(propa_stats[3], 0, "no human should have died");
+        }
+
+        stats = humans_stats(&population.humans);
+        println!("Population after propagate: {:?}", stats);
+
+        assert!(stats.infected <= infected_at_start + width * height * infected_expected);
+
+        let should_be_dead = infected_at_start * death_rate / 100;
+        let dead_tolerance = (should_be_dead as f32 * 0.20) as i32;
+
+        assert!(
+            stats.dead <= should_be_dead + dead_tolerance,
+            "death count should be less or equal than {}",
+            should_be_dead + dead_tolerance
+        );
+        assert!(stats.dead >= should_be_dead - dead_tolerance);
+    }
+}