Short answer: Ignorance.
"The Black Death was the deadliest epidemic of bubonic plague in history, wiping out some 25 million Europeans alone in just a few years. Out of desperation, cities hired a new breed of physician — so-called plague doctors — who were either second-rate physicians, young physicians with limited experience, or who had no certified medical training at all.
The costume of a plague doctor featured an all-leather ensemble, a beak-like mask stuffed with burning herbs, and a top hat — which signaled that the person was, in fact, a doctor.
What was important was the plague doctor was willing to venture into plague-stricken areas and tally the number of dead. After more than 250 years fighting the plague, hope did finally arrive with the invention of the 17th-century equivalent of a hazmat suit. Unfortunately, it didn’t work very well."
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