On this date in 1884, German physician and bacteriologist Robert Koch discovered cholera vibrio, a comma-shaped bacterium that causes Asiatic cholera, an often-deadly disease. Head of a commission investigating cholera’s causes, Koch traveled to Egypt in 1883 where an epidemic raged. He and his colleagues found that a particular bacterium existed in the intestines of those who died of cholera. But they failed to isolate the microorganism and couldn’t infect animals with the disease. Later that year Koch went to India, where he wrote that he successfully isolated “a little bent [bacilli], like a comma.” He discovered that the cholera bacteria thrived in damp, dirtied linen and moist earth and also in the stools of patients suffering from the disease’s advanced stages. Prominent Canadian physician Sir William Osler once referred to Koch as “our medical Galileo.”

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