HIV is the disease, AIDS is the result. It's contagious, not rampantly so, but anal sex is more than enough to break all the barriers to transmission. This is part of the reason why homosexuals constitute a majority of HIV infections even though they are a tiny minority of the whole population - HIV doesn't transmit nearly as well through heterosexual sex.
If anal sex is spreading a deadly disease, then why isn't anal sex illegal? (See my tag line).