Let's say we want to cure HIV. You could take some bacteria, or something that can get infected by HIV, and grow many cultures. Then, you could infect that culture with the disease. Artificially determine and kill off any bacteria which get infected. Repeat until all you have left are HIV-resistant bacteria. From these remaining, you could see what caused them to be resistant, or somehow develop a drug mimicking these properties.
We can artificially induce natural selection by (somehow) manually killing off cells which are infected. The bacteria would adapt to this environment, and essentially "learn" HIV resistance.
This assumes the following:
- We can actually kill off only the infected cells without letting them reproduce.
- We can use the resistant cells to create a drug or some sort of cure.
- The organism you use can possibly, through some mutation, become resistant to the disease. Since evolution can take a long time, perhaps something like radiation can be used to speed up the rate of mutation, and further the process along a bit.