If piss is relatively safe, shit is another matter altogether. Human excrements naturally contain any amount of pathogens. The pathogens of sexually transmitted diseases can also multiply in it.

If your taste is for shit play, you must have an vaccination against hepatitis A. This transfers itself by shit getting into the mouth. If you settle for shit contacting uninjured skin - no problem.

But all dangerous pathogens like arses. They feel very much at home in the sensitive mucous lining of the rectum - chlamydia, gonococci, syphilis, HIV and many more. So if you enjoy entering by the back door, always use a condom.