As you already seem to know, the main infections risked by analingus are
viral and bacterial intestinal infections, not traditional STDs.
In other words, if you catch anything at all, it probably would cause a combination of nausea, vomiting, and diarrhea for a few days, nothing more. However, you certainly are not "virtually guaranteed" to acquire such an infection. Maybe a 5% chance as a guess; but that's only a guess.