Well, DNA is the genetic material that determines everything. RNA, however, is what actually executes what instructions the DNA tells it to do. If the DNA tells them to create AIDS, they'll make AIDS. If the DNA tells them to create something that would fight cancer, they'll fight AIDS.
Some diseases are formed by "glitches" in the RNA, so sometimes it's hard to know who will get a disease & when. We can predict who is more likely, but sometimes things sneak up on us.
As far as cures go, it's slow going. For some diseases such as AIDS, it affects the DNA directly & becomes part of the DNA. It's difficult to find a way to kill off the AIDS virus without killing off the body's DNA that it needs to survive. Some scientists are discovering ways to manufacture the anti-disease genes that some people are going, but it's all too experimental to put out on the market. At this point there are ways to treat diseases such as this, but no way to actually eliminate it. Even chemo & drugs will only kill off existing cancer tumors & such, but it won't stop the body from making more cancer down the line.