Why do my commanders keep dying of disease?

Imagine a world where treating a disease involves leeches then wonder why disease is in it's current form.

Historically; disease killed far more people than any war did and very few if any could be stopped from running their natural course.

Even today in our modern health-conscious society we still have had recently (In history) things like Small Pox, Mlaria and others that tear through the undeveloped nations nearly unchecked.