Vaccine History Variolation:Inoculation of smallpox into skin(18th century) Vaccination:Inoculation of cowpox into skin Herd immunity results when most of a population is immune to a disease.Vaccine History • Variolation: Inoculation of smallpox into skin (18th century) • Vaccination: Inoculation of cowpox into skin • Herd immunity results when most of a population is immune to a disease