herd immunity
Chapter: 9b
The immunity of a group or community (the herd). The resistance of a group to invasion and spread of an infectious agent is based upon the resistance to infection of a high proportion of individual members of the group. If the infectious agent can't find a susceptible host to infect, then an epidemic would die out. The Sabin vaccine took advantage of herd immunity to more or less eliminate polio from North America. The vaccine strain of the polio virus could infect other members of the household, even if they had not been vaccinated.