They are not sure as there are various hypotheses apparently:

> He says vulture stomach acid is 10 to 100 times stronger than human stomach acid, "so it seems like the stomach itself is a very harsh environment."

> "Another hypothesis could be that they're actually using the bacteria in the stomach as some sort of probiotics," Hansen says. By having a gut full of a few tolerable species of bacteria, it's possible that those would crowd out other deadly microbes.