What Lolita said ...
They haven't found a single case worldwide where an individual contracted HIV via oral. They say this is because people who engage in oral - engage in vaginal sex also so it's difficult to tell. However, LOL ... there are plenty of people who engage in ONLY oral sex.
CDC says chlamydia is a possibility but then again ... there are no studies to support that claim.
Gono ... is about the only one of the serious STD's that you could catch - though it's low risk. And - I've seen studies that conclude that even if left untreated, ORAL gono will resolve within 12 weeks because it cannot survive in mouth and throat tissue.
I used to be very paranoid about this until a retired provider - who's probably the smartest person I know by the way - explained it all to me. She was VERY active in NOLA up until 2006 and probably - medium to high volume? Anyway ... from all her years of performing oral (including CIM) - she never caught any kind of STD ... ever.
We men are so stupid - we actually believe that intelligent women would take serious risks with their health for a buck? No. CIM is up to the provider (personally I can take it or leave it) ... in the German FKK's ... it's almost 100 percent for providers who allow CIM and they have no problem with STD's.
Rest assured ... all of the girls on ECCIE who've been here awhile have evaluated the risks their activities may have concerning their health. They're not going to offer you anything that they consider to be dangerous even if you're good looking like I am! Hahaha! I crack myself up!!!
EDIT: Another fun fact is that, among female sex workers - the STD rate is actually LOWER than the rate for the promiscuous population at large. If you eliminate street walkers from the numbers - it's vanishingly small.