"Rimming" is a dangerous practice:
Which has caused enteric disease by a variety of infections with Shigella species, Campylobacter jejuni, Entamoeba histolytica, Giardia lamblia, hepatitis A, hepatitis B, hepatitis non-A, non-B...
For example, shigella species are found in faeces. It is also a drinking water pathogen. Infection with shigella species results in "shigellosis", which is a dysentry caused by the varying types. Dysentry is where usually pain, fever, and severe diarrhea accompanied by blood and mucus.
Hepatitis A is known to be especially occuring among gay males that carry out this practice. Hepatitis B is known to also be able to be transmitted via this transmission route, some scientists have stated that Hepatitis B is endemic to gay male populations.