"In fact, very few Virginians were exposed to smallpox prior to the American Revolution. One of the few Virginians who was exposed was George Washington, who contracted the disease during his only trip away from the American mainland while visiting Barbados in November of 1751. Washington was only nineteen years old at the time and the illness, which lasted nearly a month, left him only with slight scarring. The brush with smallpox, however, did provide Washington with immunity from further attacks of the disease, the benefits of which would not become apparent until many years later."

Smallpox
George Washington Digital Encyclopaedia
http://www.mountvernon.org/library/digitalhistory/digital-encyclopedia/about-the-encyclopedia/