http://www.pubmedcentral.nih.gov/articl ... id=1949841
HIV causes AIDS. There are other immunodefficiency syndromes that are not caused by HIV, but HIV causes AIDS; the evidence is pretty unambiguous.
Circumcision dramatically lowers the risk of both contracting and passing on HIV, because the forskin is rich in the immune cells that are a target for that virus; normally these cells help to protect against sexually spread infections, but their presence is a weakness in the case of HIV. However, with enough risky behavior (or even just bad luck), circumcised men can still contract and pass on HIV.
Plague bacterium attacks the same receptors that HIV virus does; in areas that plague swept through, people who were naturally resistant to plague due to a mutated version of that cellular surface receptor were more likely to survive and form founder populations after the plague. People who are resistant to plague are also resistent to HIV, and survive longer with the virus than those who have normal versions of this receptor protien. European people from areas particularly hard-hit by the plague are more likely to have the mutation that makes them resistent to both diseases than European people from areas less hard-hit by the plague, but as there's been a lot of intermarrying since the black death, descent from a plagued population is nowhere near a guarantee that one isn't suceptible to HIV. There are a lot of areas in the world other than Europe that have seen plague (it's endemic in the American south west), but I haven't seen any studies on whether the people in those areas also carry resistant forms of the receptor protien in question.