Controversy: Respiratory Protection for Healthcare Workers

In the second century AD, Galen, a Greek physician to Roman emperors, observed, "When many sicken and die at once, we must look to a single common cause, the air we breathe." Miasma theory, which was prevalent from the Middle Ages to the 1800s, presumed that illness was transmitted by poisonous vapors filled with particles from decomposed matter. The belief in this theory was so strong that even after John Snow demonstrated in 1854 that cholera was waterborne, an official government investigation concluded that the epidemic was caused by vapors from the River Thames.