William Henry Harrison, the ninth president of the United States, holds a distinction that with luck will never be equaled: He was our shortest-serving president, dying on April 4, 1841, after just a month in office.

What killed him? Historians have long accepted the diagnosis of Harrison’s doctor, Thomas Miller: “pneumonia of the lower lobe of the right lung, complicated by congestion of the liver.”

The pneumonia was thought to be a direct result of a cold the 68-year-old Harrison caught while delivering a numbingly long Inaugural Address (at 8,445 words, the longest in history) in wet, freezing weather without a hat, overcoat or gloves.

But a new look at the evidence through the lens of modern epidemiology makes it far more likely that the real killer lurked elsewhere — in a fetid marsh not far from the White House.

The first clue that the pneumonia diagnosis was wrong lies in Miller’s own apparent uneasiness with it. “The disease,” he wrote, “was not viewed as a case of pure pneumonia; but as this was the most palpable affection, the term pneumonia afforded a succinct and intelligible answer to the innumerable questions as to the nature of the attack.”