1901 diphtheria antitoxin contamination incident
On October 2, 1901, a former milk wagon horse named Jim showed signs that he had contracted tetanus and was euthanized. He was used to produce serum containing diphtheria antitoxin (antibodies against diphtheria toxin).
Source: Wikipedia — 1901 diphtheria antitoxin contamination incident (CC BY-SA 4.0)