Good question. In certain diseases, such as heart disease, COPD, asthma, a perfect medicine could work indefinitely and in this situation we would no longer research the area. However, many diseases caused by viruses and bacteria get used to medicines and the medicines will become less effective or stop working altogether. Antibiotics, TB, malaria, HIV are just a few examples of where there is a constant race between these tiny micro-organisms and science. DDT was a chemical spray that wiped out over 99% of malaria spreading mosquitos, but the few that survived bred and produced more mosquitos, all of which were immune to this chemical. It is the same with viruses.