Ass-to-mouth is a colloquial term for the removal of the penis from the passive partner's anus followed by the immediate insertion of the penis into the passive partner's or another partner's mouth
It is also known as A2M, ATM, or Ass-to-Mouth. Ass-to-mouth generally excludes cleaning the penis or other object after its removal from the anus and before its insertion into the mouth. This may enhance sexual excitement for both active and passive partners, often developing from connotations of domination/humiliation. "Ass-to-other mouth," abbreviated as A2OM or ATOM, specifically describes the variant of the act where the penis is moved from one partner's anus to a different partner's mouth. Another variation is known as A2P, ATP, or Ass-to-pussy which refers to removing the penis from the anus and inserting it into the vagina. The term has been used since at least January 1995 and clearly defined since at least August 1996.[2] Despite its health risks, ass-to-mouth has become prevalent in pornographic films, depicting it as a pleasurable act.
The health risks of ass-to-mouth include the risks of both oral sex and anal-oral sex. Unprotected oral-genital contact involves the risk of contracting sexually transmitted diseases, such as genital herpes or chlamydia and HIV. HIV can be transmitted due to the fact that rough anal sex, usually results in minuscule tearing of the rectal tissue, and small amounts of blood are left on the penis and then when the performer sucks the penis, if they have even a tiny sore, tear, cut in their mouth, they can get infected with the HIV Virus. The recipient of ass-to-mouth is also at risk of infection by Hepatitis A,[5] as well as intestinal parasites and other diseases carried in the feces. Hepatitis A, according to the CDC, is only spread from a person with Hep A to another. In other words, someone can't get Hep A from performing A2M on something that has been in their own anus.
It is unknown whether thoroughly washing the anal area and applying an enema results in reduced risk of disease, although general cleanliness is advisable for any sexual act. 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.
Source - The T.V Show Doctors