h/t Plague Monk
And as even WHO has noted, your odds of contracting it drop to virtually nil if you keep other people's infected penises out of your @$$. Which is probably good advice even without the health benefits.
(FTR, I can't decide which is funnier: Plague Monk's comment, or the fact that I could Google up a picture of someone with a face mask on their butt within 0.2 seconds of first looking for it. (God is great, beer is good, and people are crazy.)