It’s not really a mystery, to me at least, when you have an understanding that, within the gay community, there are two sub-groups called “bugs” and “bug-catchers”.
“Bugs” are HIV-positive men who are actively trying to spread AIDS and the “bug-catchers” are actively trying to catch it, to gain status.
Yes, it’s messed up thinking.
I think that one turned out to be an urban legend — along the lines of the gerbil thing.