In an article titled "Bug Chasers: The men who long to be HIV+" [http://www.rollingstone.com/news/newsarticle.asp?nid=17380], writer Gregory Freemen, based on comments by public health officials, asserts that 25 percent of the 40,000 new AIDS infections every year are gay men who have sought out the disease. He also exposed how some gay men have eroticized getting AIDS as the last most extreme sex act in a community where all else has been tried.
"Bug Chasers and the Thought Police" by Tammy Bruce http://www.newsmax.com/archives/articles/2003/1/29/95042.shtml
This isn't a matter of these men not realizing they're infected--some are intentionally seeking to become HIV+, receive a medical diagnosis as such, and then continue to have unprotected sex. They are knowingly and willing perpetuating the spread of HIV.
Also they see their friends with AIDS getting hundreds of thousands of dollars worth of medications that are now keeping them alive and fairly healthy --- so AIDS is no longer a big deal to them --- they're sex addicts and AIDS doesn't kill the addiction.