HIV does mutate. My company makes one of the AIDS medicines, and has another ready for a new drug application. The older drugs (AZT, etc.) no longer work for many people, because the virus mutates to become resistant. This also makes it wicked difficult to make a vaccine.

HIV hides in immune system cells; that's why it's hard to completely eradicate. The next generation meds target these so called B-cells, breaks them open and kills the bug. (No, I don't understand how the molecule knows that a particular B cell is infected.)