Five things:
1) Crushing poverty means girls will do anything (like hook bareback...and lest we forget that Brazil suffers from a major case of Third World Catholicism, ergo some still consider condoms a sin), hence increased risk of disease exposure and transmission. This is also why you can find pics and video of AVN's current Foreign Female Performer of the Year, Monica Mattos sucking off a horse, and why 2 girls 1 cup has two Brazilian girls in it.
2) A Brazilian HIV test isn't worth the paper it is printed on. Fakes are apparently picked up very cheaply.
3) AIDS isn't exported per se, i.e. by Brazilian talent working abroad, but with the amount of foreign producers going down there to shoot (due to the shit cheapness of the talent and said talents willingness to gargle human feces and horse semen, or vice versa), it does get picked up and introduced to new talent pools in other parts of the world. If nobody went down to Brazil to shoot with the Brazilian talent, then the AIDS would stay in Brazil. Simple idea, but too few producers seem to have grasped it yet.
4) Trannies. John Stagliano got HIV from a tranny first hand, Darren James got it secondhand via Bianca Biaggi. It has always puzzled me that Evil Angel put a moratorium on its directors shooting anal creampies (due to the increased potential risk of HIV transmission), but not on shooting trannies, or going to Brazil to shoot. A classic case of putting the cart before the horse, or throwing out the baby with the bathwater, methinks.
5) Brazilians are heavily into all that UFC/MMA/BJJ stuff like that Travieso guy who used to post here...sweaty, muscular men in spandex hotpants rolling around trying to 'mount', 'dominate', and 'ground and pound' each other. If that's not gateway fudgepacker behaviour, I don't know what is.