A bug chaser refers to a gay man who wishes to contract HIV from a so called gift giver. There's been a good deal of speculation on why such behavior happens, but so far no attempt has been made to formally pathologize it. In a sensationalistic story written by Rolling Stone author Gregory Freeman the interviewee named "Carlos" seemed to do it because he had a kind of fetish for the virus and the thought of the risk and the sense of belonging to a "brotherhood" added to the level of sexual arousal. Freeman idly speculates that bug chasers see HIV infection as inevitable, an extreme sex act, or liberation from having to worry about safe sex practices.
Matt Drudge, quick to notice any story that makes queers look bad, linked to Freeman's story on the Drudge Report. This, along with a Newsweek article and a spot on Sean Hannity's conservative talk-show, brought the issue to mainstream attention more than the Rolling Stone article had. Both cited a figure that 25% of new cases of HIV were from "conscious" or "unconscious" bug chasing behavior from the original Rolling Stone article that was quoted by San Francisco director of behavioral-health services Bob Cabaj. Cabaj later denied ever even giving the figure and in hindsight that there is no real evidence that the rate is even nearly that high. However, the figure appears to have gotten a toehold in public consciousness anyway.
The general reaction to this entire mess has been mostly outrage or disgust by the majority of people and compassion from a few others. Advice columnist Dan Savage, while debunking the statistical claims made in the Rolling Stone article, railed against the poor state of AIDS prevention in the United States and the risky behavior that some groups of gay men are engaging in. The entire issue has probably received more attention than warranted and has more or less blown over without any more mention than the initial fanfare.
Sources and further reading:
http://www.rollingstone.com/features/featuregen.asp?pid=1525
http://www.advocate.com/html/stories/906/906_bugchasers.asp
http://www.villagevoice.com/issues/0305/savage.php
http://www.alternativesmagazine.com/15/hill.html
http://www.salon.com/opinion/sullivan/2003/01/24/rolling/index.html