Ass to mouth (commonly abbreviated as A2M or ATM) is a slang term primarily used to describe the withdrawal of a penis from the receptive partner's anus followed by the immediate insertion into the receptive partner's mouth. The porn industry typically employs enemas before filming the act to mitigate the associated health risks.
If the recipient of ass-to-mouth is performing fellatio on a penis or object that was removed from his/her own rectum, the health risks are generally limited to disturbances of the gastrointestinal tract, which may proceed from introducing normal intestinal flora from the rectum to the mouth and upper digestive tract. If the recipient's ano-rectal area is infected with a sexually transmitted disease like gonorrhea, however, there is an added risk of transmitting the infection to that person's mouth and/or throat. Intestinal parasites and other organisms can also be carried in feces. Risk of STI or parasitic transmission exists generally only if fecal particulate from an infected person is transmitted to the mouth of an uninfected person.
Porn industry performers often use enemas prior to filming anal sex sequences; however, this is primarily to eliminate the possibility of any fecal matter appearing on film rather than disease prevention.