Who was the first president to die in office, just 30 days after inauguration?

William Henry Harrison was the first president to die in office, just 30 days after his inauguration in 1841. His presidency was marked by a record for the shortest time served in office, largely due to his sudden death from pneumonia. Harrison's extensive inaugural address, delivered in cold, wet weather, is often highlighted; it is believed that this contributed to his subsequent illness. His untimely death led to the first instance of presidential succession, as Vice President John Tyler assumed the presidency, establishing important precedents regarding presidential succession that would be referenced in later years.