Ok, so the comment about the possibility of HIV infection from oral got me thinking. I googled the topic to find that where a woman is giving a blowjob, she would be at low risk of infection if the guy was a carrier.
The guy receiving the BJ is not at risk.
Moreover, where a guy is performing oral on a woman, he is not at risk even if she is a carrier.
I guess that means that the risk is solely to the person (of whichever sex) who is giving the BJ to a guy who is HIV positive. The risk is low (5% of cases are contracted this way), but rises if the giver of the BJ has bleeding gums or a mouth ulcer; which makes sense.
Panic over!