Pneumonia is the conventional wisdom. Stubborn ninth US president William Henry Harrison gave a long inaugural address in freezing weather, got sick, and died a month later.
What we do know, however, is that the President had a long history of indigestion. Although we have no record of how he managed his dyspepsia, the standard treatment in the 1840s was carbonated alkali, which would have neutralized the gastric acid that otherwise kills harmful bacteria.
In the absence of our natural, protective gastric acid barrier, gastroenteritis can be caused by as few as 1/10,000th the number of bacteria usually needed. And it just so happened the cause of gastroenteritis was living just seven blocks from the White House.