Eight United States presidents died while holding office, four of them at the hands of assassins. Lack of sophisticated medical technology, and a late realization for the necessity of presidential security, played a role in most of those deaths. Since the implementation of the Secret Service, only one president has been assassinated while in office. No president has died of an illness while in office for over half a century.

19th Century Assassinations

Abraham Lincoln was the first president assassinated in United States history. On April 14, 1865, at the start of his second term as 16th president, he was shot in the back of the head at Ford's Theater in Washington, D.C. The assassin, John Wilkes Booth, shouted "Sic semper tyrannis" ("thus always to tyrants") before fleeing. The 56-year-old Lincoln died across the street at William Peterson's boarding house during the early hours of April 15. Twentieth president, James A. Garfield, was four months into his first term when he was assassinated. On July 2, 1881, at a train station in Washington, D.C., Charles J. Giteau shot the president in the back. Interestingly, Secretary of War Robert Todd Lincoln -- whose own father had been felled by an assassin's bullet 16 years earlier -- was accompanying Garfield at the time. The president survived nearly four months before dying at the White House on September 19, 1881, at the age of 49.

20th Century Assassinations

William McKinley, one year into his second term as the 25th president, was shot by anarchist Leon Czolgosz while shaking hands with visitors at the Pan-American Exposition in Buffalo, New York. Though the gunshot itself wasn't fatal, the president succumbed to the resultant gangrene eight days later at the age of 58. Thirty-fifth president, John F. Kennedy, is the last president to die in office. In 1963, while the presidential motorcade moved through Dealey Plaza in Dallas, Texas, Lee Harvey Oswald fired a rifle from the sixth floor of a nearby book depository window. The 46-year-old president received a fatal gunshot wound to the head and died later that afternoon at Parkland Memorial Hospital.

Unknown Causes

The specific reason for the death of 12th president Zachary Taylor is still not specifically known. On July 4, 1850, the president celebrated the holiday by attending a ceremony to lay the cornerstone of the Washington Monument. While partaking in the festivities, he enjoyed large amounts of cherries, apples and milk. In the days that followed, the 65-year-old Taylor experienced serious digestive upset. He died on July 9. Reports at the time cited "cholera morbus" as the cause of death. Some modern scholars have attributed his fatal ailment to severe gastroenteritis, or stomach flu.

Illnesses

Ninth president William Henry Harrison was the first president ever to die in office. Thus far, he also has had the shortest presidency, lasting exactly one month. After the president caught a cold in late March -- believed to have been caused by chilly weather at his inauguration -- his condition worsened. His cold turned into pneumonia and pleurisy, or lung inflammation. Due to the demands of the office, Harrison was unable to rest enough to fully recover. Various attempts were made to cure him, including cupping, bleeding and even live snakes, but he only grew worse. On April 4, 1841, he died at the White House at the age of 68.

Heart Attack or Stroke

Warren G. Harding died two years after being elected the 29th president. Though already in poor physical health, he embarked on an extended tour of the West that included visits to Washington, Alaska and British Columbia. He ailed throughout the journey with symptoms of nausea, chest pains and a racing heart. On the evening of August 2, 1923, at the Palace Hotel in San Francisco, Harding died suddenly while talking to his wife. He was 57. Thirty-second president Franklin Delano Roosevelt, whose health had been declining over the course of his four terms in office, died during a visit to the Little White House in Warm Springs, Georgia. On April 12, 1945, the president experienced a sharp pain in his head. He then lost consciousness, dying later that afternoon at the age of 63.