This practice runs on consent that either partner can pause or end at any time, in either role — and if you have anal-area trauma or medical history, not exploring is itself complete self-protection.
Oral-anal contact can pass on several infections: STIs like herpes, HPV, syphilis, and gonorrhea, plus intestinal infections such as hepatitis A, shigellosis, and giardiasis. The risks are not equal — HIV transmission via oral routes sits at the very low end of the sexual-exposure spectrum, so it shouldn't be lumped in as an equal-scare item. Much of the intestinal-infection evidence comes from clinical cohorts and outbreak networks among men who have sex with men; the transmission routes themselves apply generally.
A barrier (dam or cut condom) may reduce these risks — that's theoretical protection plus guideline recommendation, not trial-proven. If either of you has an active sore — mouth or anal area — wait until it heals. With bleeding hemorrhoids, a fissure, or active inflammatory bowel disease, check with a clinician first.
One more hygiene rule: moving mouth or fingers from the anus to the vulva or urethral area can carry intestinal bacteria toward those areas, which is associated with bacterial vaginosis and urinary tract infections. Staying one-directional, changing a barrier, or washing hands between areas is considered a reasonable precaution — it lowers odds, it doesn't block them.
This rundown exists so you can decide with real information — not to scare you into or out of anything. Pain, bleeding, or lasting discomfort is a stop, and a repeat pattern goes to a clinician.