If the girl has an infection in her mouth, it is possible. Say she blew a guy with a Chlamydial infection in his urinary tract, she got it in her mouth, then she blows you and you now are at risk for contracting it. If the girl has the bacterial infection in her urinary tract, you won't get it from receiving oral sex, thats common sense, the infection isn't in her mouth.
It seems people think oral sex isn't really "sex" and it is safe under any circumstance, especially the guy who gets his dick sucked. But think about it, there is a lot of bacteria that can infect a mouth, do you really want your dick in there. And ladies (and guys too if that's your thing) your mouth is an entrance to your body, putting strange cocks in there is no safe practice.
My point is guys be selective, quality over quantity. And girls and gay bros, know who you blow.