Illustration of a woman who died of cholera. Cholera, a bacterial infection of the intestines, was often fatal before modern treatments were developed. The skin of victims had a blue pallor due to the deoxygenated and dehydrated blood thickening under the skin. It spread in infected water supplies and killed millions in the 19th century. The bacterium causing the disease (Vibrio cholerae) was not identified until 1883. Coloured lithography by John William Gear (1832).