C,
We've witnessed it first hand twice with our son. He and his fiance' were in SP for five days last Christmas, and the combination of B Complex (double dose for two weeks prior) and a good DEET spray did the trick. Two bites in five days. Apparently the B complex gives the skin a bit of a sulpur treatment that the little rascals don't like. Fortunately they do not bother me, maybe two bites in ten years, but the rest of this group gets it pretty bad.