I don't know how you can attribute a purpose to AIDS. To say that evolution is the purpose of AIDS, which is the only thing that can fit the definition of purpose that Midnight gave is ludicrous. AIDS exists because it is the unguided consequence of random genetic variance in previous virus', that exist because of random molecular changes in chemical reactions, which exist because of random fluctuations in the cooling of suns, which exist because of the big bang. We could equally say that the purpose of AIDS is the Big Bang. Does that make sense to you? Of course it doesn't, because it's not what we mean by "purpose." AIDS has no purpose.
I believe you are the parrot dealing with "aids and evolution".
I think the problem lies in that the definition of 'evolution' is being used to fit whatever idealism is being suggested. Some take it as "life evolves as a purpose to change and survive". Some take it as "things that change do so regardless of their station on the planet".
AIDS IS an evolved lifeform. Mutations in viral forms of life are almost always seen for what they are, adaption, a direct correlation to Evolutionary Theory. It did so by mutation, through the jump in vectors from animals to man, so literally speaking, it's fulfilled a purpose, again not divine or man-made, and that "purpose" was to swap genetic pools. It did so, and thus a purpose was fulfilled. Arguing against that is, well, ludicrous.
Why does it rain? To fulfill the purpose of weather patterns. There is no "opinion" about it in aforementioned clouds, there is merely the process. Thus, a purpose. It really is not that complicated.
I think you are getting purpose of things confused with "man-made" notions of importance versus non-importance. Again, a purpose is just a purpose, not some lofty grasped concept only man can take hold of.