For example - If you get some of your own feces in your mouth, because your fingers were in your ass, or your partner's dick was in your ass then pulled out and you performed oral sex on him, you could contract Hepatitis or get an e-coli infection - from yourself!

It doesn't matter that it is YOUR feces (or other rectal fluids), it is that the mouth and stomach cannot protect against Hepatitis or e-coli period.

And, there's other ways in which infection can enter the body - though cuts, sores, tears, abrasions or other weak points in the skin.