Oral sex eating ass. When you ingest someone else's gut biome there are things (think herpes) that passes the blood brain barrier. Could be contracted from poop food poisonings as well. But mostly eating asshole even a teeny little lick. Well my grandparents only had sex with each other and only missionary no oral. But you don't know that. No doctor in the world asks have you licked a butthole. Did your mate suck your dick after you had sex and your dick was near the butthole? Have you ever eaten someone out and gone home and kissed your spouse? They don't ask shit like this and it skews any data because they don't. Have you ever had food poisoning? Uh oh. Poop got in your mouth. Polio. Poop. Alzheimer's poop.