Cholera
Cholera  was a bacterial disease that affected Humans. It was prevalent on Earth in the 19th centuries. It was spread by contaminated water. Though lethal, it typically did not kill large numbers of people at a time, as it was not virulent enough to do so.

The Devidians used a cholera outbreak in 19th century San Francisco as a cover for their killing of Humans to harvest neural energy. ()

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Medical conditions