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Rimming
It is not safe to get shit in the mouth or the eyes: although it doesn't normally contain HIV there can be blood in it sometimes, which does contain HIV. But there is also a high risk of transmitting a large range of other organisms (including viruses, bacteria, and parasites). These can all cause very serious diseases in their own right, and can be especially severe in someone with HIV infection.
Rimming even a clean arsehole is potentially risky as there are some extremely infectious microscopic organisms which ordinary hygiene cannot be guaranteed to deal with. But obviously the risks are even higher if you rim someone who's recently had diarrhoea, for instance.
Barrier rimming
If you wish to use a barrier for rimming, dental dams or clingfilm may be used. See Dental dams for further information.