If you cure a disease you get a, lump sum at discovery when you distribute it. You then lose profit as rates of cute needed go down and eventually are non existent. On the other hand treatments for diseases such as Aids insure for most, an average life span in which thier healthcare now in abundance pays a, hefty portion of treatment.
It is true that the incentive of most main, direct funders of medical researcher is to make money. That is why drugs are typically patented, and the prices are kept high. It would make sense, therefore, that if cures to diseases were found, they would be pushed under the rug in order to make room for the money that treatments would make the funders. After all, money makes the world go round.
Truth be told, there is prevention, really, but not cures for diseases. It's far easier (although, any research is difficult and takes many years) to treat the symptoms of a disease than to cure a disease completely. The best success we have had thus far is in prevention, such as with smallpox and polio vaccines. That seems to be the best way we have found to combat disease thus far. If one were to find a cure for a disease, it would be extremely profitable, and would bring prestige and more research dollars to the researchers who discover it. This debate question is an argument often made, but it's one that doesn't seem to have much merit based on what we know about disease.