remove casts and correct 0 limit situation on roll()

src/disease.rs

@@ -5,14 +5,14 @@
 
 // #[derive(Debug)]
 pub struct Disease {
-    pub infection_rate: u32,
-    pub curing_rate: u32,
-    pub death_rate: u32,
-    pub traits: Vec<u32>,
+    pub infection_rate: i32,
+    pub curing_rate: i32,
+    pub death_rate: i32,
+    pub traits: Vec<i32>,
     pub name: String,
 }
 impl Disease {
-    pub fn new(infection_r: u32, curing_r: u32, death_r: u32, the_name: String) -> Self {
+    pub fn new(infection_r: i32, curing_r: i32, death_r: i32, the_name: String) -> Self {
         Self {
             infection_rate: infection_r,
             curing_rate: curing_r,