<P> Herd immunity (also called herd effect, community immunity, population immunity, or social immunity) is a form of indirect protection from infectious disease that occurs when a large percentage of a population has become immune to an infection, thereby providing a measure of protection for individuals who are not immune . In a population in which a large number of individuals are immune, chains of infection are likely to be disrupted, which stops or slows the spread of disease . The greater the proportion of individuals in a community who are immune, the smaller the probability that those who are not immune will come into contact with an infectious individual . </P> <P> Individual immunity can be gained through recovering from a natural infection or through artificial means such as vaccination . Some individuals cannot become immune due to medical reasons and in this group herd immunity is an important method of protection . Once a certain threshold has been reached, herd immunity gradually eliminates a disease from a population . This elimination, if achieved worldwide, may result in the permanent reduction in the number of infections to zero, called eradication . This method was used for the eradication of smallpox in 1977 and for the regional elimination of other diseases . Herd immunity does not apply to all diseases, just those that are contagious, meaning that they can be transmitted from one individual to another . Tetanus, for example, is infectious but not contagious, so herd immunity does not apply . </P> <P> The term herd immunity was first used in 1923 . It was recognized as a naturally occurring phenomenon in the 1930s when it was observed that after a significant number of children had become immune to measles, the number of new infections temporarily decreased, including among susceptible children . Mass vaccination to induce herd immunity has since become common and proved successful in preventing the spread of many infectious diseases . Opposition to vaccination has posed a challenge to herd immunity, allowing preventable diseases to persist in or return to communities that have inadequate vaccination rates . </P> <P> Some individuals either cannot develop immunity after vaccination or for medical reasons cannot be vaccinated . Newborn infants are too young to receive many vaccines, either for safety reasons or because passive immunity renders the vaccine ineffective . Individuals who are immunodeficient due to HIV / AIDS, lymphoma, leukemia, bone marrow cancer, an impaired spleen, chemotherapy, or radiotherapy may have lost any immunity that they previously had and vaccines may not be of any use for them because of their immunodeficiency . Vaccines are typically imperfect as some individuals' immune systems may not generate an adequate immune response to vaccines to confer long - term immunity, so a portion of those who are vaccinated may lack immunity . Lastly, vaccine contraindications may prevent certain individuals from becoming immune . In addition to not being immune, individuals in one of these groups may be at a greater risk of developing complications from infection because of their medical status, but they may still be protected if a large enough percentage of the population is immune . </P>

How is herd immunity affected by low-contagion and highly-contagious diseases