In the early 1800s, a medical student poured the black vomit of dying yellow fever patients into his own eyes, smeared it into open cuts, inhaled its fumes, and even drank it undiluted. And he walked away completely unharmed. It is one of the most grotesque acts of self-experimentation in the history of science.

This episode profiles Stubbins Ffirth, an American trainee doctor shaped by the 1793 Philadelphia yellow fever epidemic that killed roughly 10% of the city. We follow how his bravery, rigorous logic, and a single missing variable led him to a confident conclusion that was utterly wrong, delaying the truth for six decades and revealing the dangers of confirmation bias.

• How the 1793 Philadelphia epidemic killed around 5,000 people, about 1 in 10 residents
• Why Ffirth's winter case-drop observation was correct but his ...