Contact immunity is the property of some vaccines, where a vaccinated individual can confer immunity upon unimmunized individuals through contact with bodily fluids or excrement. In other words, if person "A" has been vaccinated for virus X and person "B" has not, person "B" can receive immunity to virus X just by coming into contact with person "A". The term was coined by Romanian physician Ioan Cantacuzino.
The potential for contact immunity exists primarily in "live" or attenuated vaccines. Vaccination with a live, but attenuated, virus can produce immunity to more dangerous forms of the virus. These attenuated viruses produce little or no illness in most people. However, the live virus multiplies briefly, may be shed in body fluids or excrement, and can be contracted by another person. If this contact produces immunity and carries no notable risk, it benefits an additional person, and further increases the immunity of the group.
