French Chemist Louis Pasteur

In 1879, Louis Pasteur made a groundbreaking discovery that laid the foundation for modern vaccines: he developed the first attenuated (weakened) vaccine using the chicken cholera bacterium (Pasteurella multocida).

Background:

Before this, Pasteur had already made significant contributions to microbiology, including work on fermentation and germ theory. In the late 1870s, he turned his attention to animal diseases—specifically chicken cholera, a deadly illness in poultry that caused severe economic losses.

Discovery of Attenuation: (Weakened Pathogens)

• In 1879, Pasteur and his team were culturing the chicken cholera bacteria for experiments. They accidentally left a culture sitting unused over the summer. When they later injected chickens with this aged culture, the chickens did not die—they became ill but recovered.
• They then injected these same chickens with a fresh, virulent culture, and the chickens survived. This indicated they had become immune.
• This was a critical moment - The aged bacteria had lost their virulence but still stimulated immunity—a process later termed attenuation.

What Is an Attenuated Vaccine?:

An attenuated vaccine uses a weakened form of the pathogen that is no longer able to cause disease but can still provoke an immune response. This principle is now used in vaccines for diseases like measles, mumps, rubella, and yellow fever.

Pasteur's Method:

1. Cultured bacteria from infected chickens
2. Allowed cultures to age (exposure to oxygen and air weakened them)
3. Injected aged cultures into healthy chickens → mild illness or no illness
4. Later injected fresh bacteria into these same chickens → no illness = immunity