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How does herd immunity reduce the spread of infectious diseases?

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Immune stimulation by activated helper T cells — immune system Stimulation of immune response by activated helper T cells. Activated by complex interaction with molecules on the surface of a macrophage or some other antigen-presenting cell, a helper T cell proliferates into two general subtypes, T_H1 and T_H2. These in turn stimulate the complex pathways of the cell-mediated immune response and the humoral immune response, respectively.

Herd immunity occurs when a sufficiently large proportion of a population becomes immune to an infectious disease, either through vaccination, natural immunity, or previous exposure. This state reduces the spread of disease by lowering the density of susceptible individuals, which minimizes the likelihood that an infected person will come into contact with someone who can contract and transmit the pathogen.12

By breaking the chain of transmission, herd immunity effectively limits the disease's ability to spread throughout a community. This provides vital protection to vulnerable individuals who cannot be vaccinated or who remain susceptible despite immunization. The specific threshold of immunity required to achieve this effect varies depending on the contagiousness of the pathogen and the social dynamics of the population. When this threshold is met, the pathogen lacks sufficient hosts to sustain an outbreak, which has historically been critical in controlling or eradicating diseases such as smallpox and polio.32