Bear in mind that the reason thongs aka G-strings aka Bacteria Highway causes infection it is because of how it is worn, Oh yea it's sexy and all that showing butt cheeks, no panty line. If you've worn or realize that the back part of the thong actually slits into the butt crack, this is why it's a highway for bacteria to the groin region. I don't understand how people wear thongs though.

A friend once commented that how occasionally the normal undies rides up into the crack and it is already uncomfortable. Thongs are in that position 24/7.

With regards to health versus underwear, I honestly don't see any relations between the two. Bras perhaps has, due to the fact that they are near lymphatic nodes but not the other.