Botulism was a form of disease that scavenging dinosaurs got from rotting carcasses.

The Great Survivors: Botulism is first implied when a Zunityrannus backs down from a Nothronychus. The mid-sized tyrannosaur then proceeds to feast on a carcass near a toxic lake. The lake was made toxic by the carcass, not the other way round. Anyhow, the Zunityrannus that ate the carcass was later seen dead by the lakeside. Three Zunityrannus' that had failed to take down some Nothronychus then cannibalised the Zunityrannus carcass. They were also seen dead later in the episode. The narrator explained that rotting carcasses were a magnet for botulism. Any animal who ate the infected carcass would immediately be infected themselves. The disease was almost always fatal, as proved in the episode. The narrator also stated that the Nothronychus were mostly safe, because they were plant-eaters. Unless they drank from the toxic lake, of course.