The chances of transmitting HIV from an infected person to an uninfected partner during oral sex is quite low, but this does not mean that it might not happen. Eating cum increases the risk. However, swallowing the cum is more advisable
Just by taking his cum in your mouth, you could contact the disease even if you do not swallow. If there are cuts, ulcers any form of sores or abrasions in your mouth, this can always lead to transmitting the disease easily. Actually, once the semen gets into the stomach, acids in your stomach are capable of killing the virus.
Apart from HIV, there are other sexually transmitted diseases which could be transmitted during oral sex. However, the risk of getting any disease during oral sex is far lower than when you are doing anal or vaginal sex without making use of a condom.
People often ask about how to kill the disease before swallowing, but most people really cannot wait that long. If he cums in your mouth, just swallow it because the acids in your belly would help kill the virus totally. So it is safer and more advisable to swallow the cum instantly once you have it in your mouth already.
However in a case where you have cuts, bruises, ulcer or abrasion in your mouth, always ensure you abstain from having cum in your mouth, because it is more risky.
For a situation where you cannot avoid taking the semen in your mouth probably because he is holding your head tightly to his crotch, and you have any bruises or cuts in your mouth, it is more advisable to swallow just to be on a safer side. However