Sentence:  At any given time, there are often dozens of people actively posting online ads as “<WSD>chasers</WSD>” or “gifters” in San Francisco and elsewhere around the nation .. Though <WSD>chasers</WSD> 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 <WSD>chasers</WSD> 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. 
Meaning of the word marked with <WSD> tags:  A person who seeks partners with HIV in order to become infected with HIV.