The number originates from a Rolling Stone article titled Bug Chasers by Gregory Freeman:
Cabaj estimates that at least twenty-five percent of all newly infected gay men fall into that category.
Cabaj disputes having said this:
That's totally false. I never said that. And when the fact checker called me and asked me if I said that, I said no. I said no. This is unbelievable.
Apart from Cabaj, the Rolling Stone also attributed claims to Marshall Forstein, who also disputes the quotes (same source as before):
That is entirely a fabrication
The Guardian spoke to a number of physicians who call the claims in the article a fabrication.
BBC investigated the claim and concludes that bug chasing is a fantasy that exists online, but that is not executed in real life.
Parsons and Grov studies the issue and conclude:
These data suggest bug chasing and gift giving do exist; however a sizable portion of both bug chasers and gift givers were not intent on spreading HIV.