The belief that pneumonia killed president William Henry Harrison was reached more out of hindsight than analysis of his symptoms. According to Oxford Academic, it was rationalized that since his inauguration was spent outdoors in cold and rainy weather without protective clothing, Harrison must have contracted it. Per descriptions of his final days, however, the president had great abdominal pain and severe intestinal issues in addition to pneumonia-like symptoms. Given that the cholera-related deaths of Zachary Taylor and James Polk only came about a few years after Harrison’s, it is possible they were all killed by the same thing (via JamesPolk.com).

These three presidencies occurred at a time before Washington D.C. had a modern sewer system. Construction on one began in 1810, but it took much of the 19th century to complete (via the District of Columbia Water and Sewer Authority). Without a more effective means of waste removal than dumping it in nearby rivers and lakes, it is believed that waste material became mixed with the White House’s potable water supply (via History Is Now Magazine). Since this is the ideal environment for a variety of bacteria to grow in, the three presidents may have died in part or solely due to drinking them, inducing gastroenteritis and later cholera.