vultures are extraordinarily efficient scavengers, eating nearly all of a carcass less than an hour after finding it, and farmers in India historically relied on them to quickly remove livestock carcasses, so they functioned as a natural sanitation system in helping to control diseases that could otherwise be spread through the carcasses they consume. In 1994, farmers began using diclofenac to treat their livestock, due to the expiry of a patent long held by Novartis leading to the entry of cheap generic brands made by Indian companies. Diclofenac is a common painkiller, harmless to humans, but vultures develop kidney failure and die within weeks of digesting carrion with even small residues of it. …
When the vultures died out, far more dead animals lay around rotting, transmitting pathogens to other scavengers like dogs and rats and entering the water supply. Dogs and rats are less efficient than vultures at fully eliminating flesh from carcasses, leading to a higher incidence of human contact with infected remains, and they’re also more likely to transmit diseases like anthrax and rabies to people. Sudarshan and Frank estimate that this led to ~100,000(!) additional deaths each year from 2000-05 due to a +4.2%(!) increase in all-cause mortality among the 430 million people living in districts that once had a lot of vultures, which is staggering; this is e.g. more than the death toll in 2001 from HIV/AIDS (92,000), malaria (53,000), and alcohol use disorders (14,000).
To lazily quote myself in case helpful: