Herd immunity is a concept that comes from the field of zoology. It refers to the ability of a population of animals to resist infection by a pathogen — such as a virus — because a sufficiently large number of individuals within the population have humoural immunity on an individual level. Humoural immunity is the ability of the immune system to form antibodies against a specific infectious agent.
With herd immunity, transmissibility in a population is dramatically reduced via immunological mechanisms. This is the theory behind vaccines, which boost specific immunity within (ideally) a very large proportion of the population, such that a transmissible disease never gains a foothold.
— Read on www.realclearscience.com/articles/2020/03/03/how_to_build_behavioral_herd_immunity_against_covid-19_111309.html