Vultures feed on carrion at stages of decay that other vertebrates avoid. Stomach pH in Gyps vultures is reported to be around one or lower, acidic enough to destroy most vegetative bacteria and many of the toxins they produce before they reach the intestine.
Bacterial spores are a different matter. Genomic surveys of vulture gut communities find the surviving flora dominated by Clostridia and Fusobacteria, taxa associated with decomposition and, in other animals, with disease. Spore-formers pass the acid barrier rather than being destroyed by it, which is part of why the hindgut looks the way it does; the vultures appear to tolerate them, and this filtering role is why vulture populations are described as reducing the circulation of carcass-borne pathogens.
Chemical resilience is not general immunity. The collapse of South Asian Gyps vultures from the late 1990s, exceeding ninety per cent in some populations, was caused by the veterinary drug diclofenac in cattle carcasses, which causes kidney failure in vultures. A gut that handles rotting meat offers no protection against a pharmaceutical residue.