The ravages of smallpox were lessened in the 1700s because Onesimus, a slave to Cotton Mather in Boston, introduced an African vaccination practice that had made his body immune to the smallpox virus. Dr. Zabdiel Boylston of Boston inoculated some 240 people, following Onesimus's description of infecting healthy people to establish an immune reaction to the virus; only six came down with smallpox. During the American Revolutionary War this method of inoculation was used to prevent soldiers from contracting the disease.