For most of history, the best trained doctors blamed disease on miasma, a word tracing back to the ancient Greek for pollution: foul vapors rising from swamps, garbage, and rotting matter. This episode follows the long fight to replace that idea with germ theory. Thucydides saw people catch the plague of Athens from nursing the sick around 400 BC, and around 36 BC the Roman Marcus Terentius Varro warned of minute creatures, too small to see, that float in the air and enter through the mouth and nose. In 1668 Francesco Redi’s gauze covered jars of meat undercut spontaneous generation, and Antonie van Leeuwenhoek’s microscopes of the 1670s revealed animalcules in pond water and tooth plaque.

Seeing microbes was not the same as proving they caused disease, and the resistance was fierce. In 1847 Ignaz Semmelweis cut deaths in a Vienna maternity ward from about 18 percent to about 2 percent with chlorine handwashing, and was mocked and forced out. John Snow mapped the 1854 Soho cholera deaths to the Broad Street pump, Louis Pasteur’s swan neck flasks showed that air alone does not spoil broth, Joseph Lister brought carbolic acid into surgery, and Robert Koch set out his four postulates in 1884.

• Athanasius Kircher saw little worms in plague victims’ blood in Rome in 1656 and urged quarantine, burning clothes, and face masks
• Why miasma loyalists saw microbes in the sick as a product of decay, not its cause
• Louis Daniel Beauperthuy blamed yellow fever on a striped legged mosquito in the 1850s and saw his work dismissed
• How the bend in Pasteur’s flask neck trapped airborne dust and microbes before they reached the broth
• Where Koch’s postulates break down: asymptomatic carriers, viruses that need living cells, and prions