Whereas this did not occur, it does have an origin; in response to the movie’s writer-director, the late John Singleton, it was really a joke (not probably the most tasteful joke, I do know) he began after they had been filming. The backstory is that each he and Tupac had HUGE crushes on Jackson (who was not all for both of them, ‘trigger she was secretly married on the time), and he would jokingly say, “We’re gonna need to get you an AIDS check so that you can kiss my actress!” as a solution to one-up Tupac. The joke grew to become a rumor even earlier than the film was launched.
Additionally, it is necessary to notice that HIV/AIDS had been a identified illness for over a decade-plus when the film was filmed — it was already a identified proven fact that you would not transmit the illness via kissing, so it does not even make sense.