Muppets have a way of relating to people, especially kids, on a level few other things or people do. Can a Muppet actually read? Can Gonzo actually pick up a phone without it being glued to his hand (as in the Jim Henson Hour episode)? Kami is a way to inform kids in South Africa of an issue that plagues them every day - and she does it in such a way that is thoughtful instead of scary.
Also, I believe it is explained that Kami got the disease from her mother. It is explained as a blood disease (which it technically is) rather than anything to do with sex. Kami teaches South African kids how to love, hug and be friends with other kids infected with HIV and how to keep safe when someone scrapes a knee while playing etc.
However, if this concern was meant out of mostly silliness, please disregard this post.