Inoculation refers to the injection of dead or weakened disease-causing bacteria or viruses into the human body in order to produce immunity against that disease. The use of inoculation to prevent disease most likely started with smallpox, a dreaded disease first named variola, in 570 A.D. In the ninth century, smallpox was differentiated from measles by a noted Persian physician, Rhazes. Historical accounts mention that epidemics of smallpox raged in many parts of the world.

It is thought that perhaps, thousands of years ago in China, doctors would remove pus and scabs from people who were suffering from smallpox. They would put the mixture into scratches made on a healthy person's arm, a procedure later called variolation or scarification. Then the healthy person would get the disease, only mildly, and would thereafter have immunity against smallpox. A problem arose some of the time when the inoculated person contracted...