At any given time, there are often dozens of people actively posting online ads as “chasers” or “gifters” in San Francisco and elsewhere around the nation .. Though chasers and gifters are active around the world, many see San Francisco as a kind of mecca .. apps like Grindr and Recon, as well as the website Breeding.Zone, where gifters and chasers share advice and stories about their sexual experiences, make it relatively straightforward to meet people who want to be infected with the virus that causes AIDS — or to infect their partners.