The Chicken Cholera Attenuation Discovery

In summer 1879, Pasteur's assistant Charles Chamberland prepared to test chicken cholera cultures and discovered the flasks had been left standing for weeks over the vacation. He injected the old cultures anyway. The chickens survived. When subsequently inoculated with fresh virulent cultures, they survived that too. Pasteur recognized — not explained, not yet understood — that the old cultures had been transformed into something protective. The recognition came before the mechanism. Over the following year, Pasteur worked out the principle of attenuation: virulent organisms could be weakened by environmental exposure to the point where they lost their power to cause disease while retaining the power to confer immunity. The principle became the foundation of all subsequent vaccination — anthrax, rabies, and eventually the hundreds of vaccines that transformed public health across the twentieth century.