One doctor proved a germ causes disease by tracking anthrax through generations of mice

In 1876, Robert Koch isolated anthrax bacteria from infected cattle, grew them outside a body, then injected them into healthy mice, who developed the same disease. He repeated the transfer generation after generation with the same result every time, turning germ theory from a hypothesis into a demonstrable chain of cause and effect. The method became the template for identifying infectious disease ever since.

— Robert Koch, Die Ätiologie der Milzbrand-Krankheit — Beiträge zur Biologie der Pflanzen, 1876