Which US President died of pneumonia while in office?

On March 26, 1841, William Henry Harrison became ill with a cold. According to the prevailing medical misconception of that time, it was believed that his illness was directly caused by the bad weather at his inauguration; however, Harrison's illness did not arise until more than three weeks after the event. The cold worsened, rapidly turning to pneumonia and pleurisy. He sought to rest in the White House, but could not find a quiet room because of the steady crowd of office seekers. His extremely busy social schedule made any rest time scarce.

Harrison's doctors tried cures, applying opium, castor oil, leeches, and Virginia snakeroot. But the treatments only made Harrison worse, and he became delirious. He died nine days after becoming ill, at 12:30 am on April 4, 1841, of right lower lobe pneumonia, jaundice, and overwhelming septicemia. He was the first United States president to die in office. His last words were to his doctor, but assumed to be directed at John Tyler, "Sir, I wish you to understand the true principles of the government. I wish them carried out. I ask nothing more." Harrison served the shortest term of any American president: March 4 – April 4, 1841, 30 days, 12 hours, and 30 minutes.