ANSWER:
We believe that Allah causes everything in this world by His will and if He wills it, no one would get infected or sick! However, He also ordered us to run away from people with leprosy like we run away from a lion!
We are ordered to stay away from infected places however, there are people who come in contact with sick people, yet do not get infected which means that illness and disease move according to Allah’s will and not by themselves.