Why are people prone to so many illnesses?

Every kind of illness you can think of, people have suffered through. Some people had them because of staying out too long in the sun (ex melanoma) others got diseases randomly. Why does this happen? I just feel that God (if he exists) wouldn't let this kind of horrid occurence to exist. Ive known so many good people who have gotten sick with horrible diseases for no reason, some were totally healthy. I wish no one would get sick and suffer through all of these illnesses.