Sheep were the main animals used in Louis Pasteur’s famous public trial of the anthrax vaccine in 1881.
The experiment took place at Pouilly-le-Fort near Melun in May 1881. Pasteur’s team divided animals into vaccinated and unvaccinated groups. The vaccinated animals received two injections, followed by exposure to a highly virulent culture of anthrax bacteria. The unvaccinated animals died or were dying, while the vaccinated animals survived, creating a dramatic public demonstration of protective immunization.
The trial included more than sheep: historical accounts describe two groups of 25 sheep, along with one goat and several cattle. That broader lineup explains why “sheep” is the canonical quiz answer while the other animals are not entirely unrelated to the event. Sheep were the most prominent and numerous test animals, and anthrax was a major threat to sheep, cattle, and other livestock.
The experiment helped make vaccination against animal diseases widely accepted. Later historical research also showed that Pasteur’s laboratory notebooks complicated the public story about exactly how the vaccine had been prepared.