Herd immunity describes a state in which an infectious disease transmissible through human contact is unlikely to spread because a large proportion of the population is immune to the disease. This immunity can be conferred through natural immunity, previous exposure to the disease, or vaccination. The entire population does not need to be immune to attain herd immunity. Rather, the population density of persons susceptible to infection must be low enough to minimize the likelihood of an infected individual coming in contact with a susceptible individual. Herd immunity can prevent sustained disease spread in populations, thereby protecting susceptible individuals from infection. It is important to emphasize that the concept of herd immunity is applicable only to infectious diseases that can be spread by human contact. ...