A means of protecting a whole community from the spread of an infectious disease.
The more people (a herd) that are immune from a disease, the better protected the entire community is from an outbreak of that disease. The most common modern way to achieve herd immunity is through vaccination. Each disease, depending upon how they spread, has a threshold, or the minimum number of individuals that need to be immunized to prevent an outbreak. Measles, for example, requires 95 percent of the population to be immunized to prevent an outbreak.