Herd Technology

In order for an infectious disease to occur in epidemic form, the causative microorganism must be transmitted easily from one susceptible host to another within the population. Unless a sufficient proportion of the population is susceptible, the disease can occur only in an endemic or sporadic form. For instance, if 70 percent of Schoolchildren in a population are immunized against poliomyelitis, epidemics of this disease are unlikely to occur even among the remaining 30 percent who were not immunized. These latter children enjoy what has been termed herd immunity. This is not true immunity but merely an expression of the unlikelihood that a susceptible individual will encounter the causative agent of the disease