After giving the Civil War speech, Lincoln became ill with symptoms of smallpox: high fever, weakness, severe pain in the head and back, "prostration" — an old-fashioned word for extreme fatigue — and skin eruptions that lasted for three weeks in late 1863.
Lincoln’s doctors told the ailing president he suffered from a cold or a "bilious fever" before one physician told him he had a mild form of smallpox.
"Lincoln’s physicians attempted to reassure him that his disease was a mild form of smallpox, but that may have been to prevent the public from fearing that Lincoln was dying," said Dr. Armond Goldman, emeritus professor of pediatrics at the University of Texas Medical Branch in Galveston.
Smallpox, which was eradicated in 1979, was widespread in the 1800s and killed 30 percent of first-time victims.
There was a very crude vaccine, but few people were immunized in the 19th century. Those who were immunized could become infected, but with a mild form of the disease. Historians had assumed that Lincoln had this mild form and had been immunized.
But Goldman and co-researcher Frank Schmalstieg studied descriptions of Lincoln’s symptoms. It appears he had the more severe infection that suggests he had not been immunized, they reported in the Journal of Medical Biography.
"His death due to smallpox would have undoubtedly changed the subsequent history of the country," Goldman said in a statement.
"At the least, the goals that were attained during the rest of Lincoln’s presidency would have been obtained less rapidly and perhaps less completely."
Lincoln delivered his Gettysburg Address on the site of a Pennsylvania battlefield in November 1863. In 1864, under Lincoln’s leadership, U.S. armies began decisive campaigns against Confederate forces leading to the collapse of the rebellion by southern states in spring 1865.
Lincoln died on April 15, 1865, after being shot the night before by actor John Wilkes Booth.
Smallpox, eradicated after a global vaccination campaign, remains the only human disease to have been fully eliminated by vaccination.
Samples of the virus remain under lock and key however, and vaccination of the U.S. military and some key health and emergency workers has been resumed because of fears the virus could be used as a biological weapon.