His symptoms first began during his train ride to Gettysburg on November 18, when Lincoln reportedly told his private secretary, John Hay, that he felt dizzy and weak.
Looking pale and still feeling sick, Lincoln pressed on and successfully delivered his famous speech without a hitch.
On the train ride home, his symptoms worsened and he began to complain of a severe headache.
Back at the White House, Lincoln soon developed back pains, high fever and a widespread rash that eventually morphed into itchy lesions.
According to the autobiography of surgeon J.M.T. Finney, after losing a large amount of weight and feeling unwell for weeks, a doctor finally diagnosed his illness as a mild form of smallpox that later came to be known as Variola minor.
Yet, a 2007 study published in the Journal of Medical Biography suggests that Lincoln could not have had Variola minor, and instead had the life-threatening form of smallpox known as Variola major.
According to the study, Variola minor didn’t even appear in the U.S. until the end of the 19th century and was unknown in the U.S. at the time of Lincoln’s diagnosis.
The study suggests that Lincoln’s doctor may have tried to downplay the serious of his illness to prevent panic among the public.
It is unknown how Lincoln may have contracted smallpox, but it was a widespread and highly contagious disease at the time.
Despite the fact that smallpox vaccines were commonly used to prevent contracting the disease, there is no evidence that Lincoln was ever vaccinated against it.
After feeling ill for three weeks, Lincoln eventually made a full recovery and resumed his presidential duties.
“A Surgeon’s Life: The Autobiography of J. M. T. Finney”; J. M. T. Finney
MSNBC: Did Abe Lincoln Have Smallpox?: http://www.msnbc.msn.com/id/18727435/ns/technology_and_science-science/t/did-abe-lincoln-have-smallpox/#.TyRokYFSySo
Abraham Lincoln’s Gettysburg Illness: http://www.ncbi.nlm.nih.gov/pubmed/17551612