The biggest myth I've ever heard about HIV: Well, actually, I've heard it in the mainland [China], when I do preventive things. In the villages, they believe that by shaking hands, or just sharing farm tools, they will get the HIV virus. And because of this kind of misunderstanding and myth, they're OK to expel the family members from the village.
I'm glad that there are some people doing things, and I hope that it will gradually change the attitude. But that's the greatest myth I've ever heard from my work.