Individuals who are immune to a disease act as a barrier in the spread of disease, slowing or preventing the transmission of disease to others. An individual's immunity can be acquired via a natural infection or through artificial means, such as vaccination. When a critical proportion of the population becomes immune, called the<unk>  threshold (HIT) or<unk>  level (HIL), the disease may no longer persist in the population, ceasing to be endemic. The theoretical basis for<unk>  generally assumes that vaccines induce solid immunity, that populations mix at random, that the pathogen does not evolve to evade the immune response, and that there is no non-human vector for the disease.
Herd immunity
Contact immunity
Adoptive immunity
Trained immunity
Passive immunity
Host adaptation
Original antigenic sin
Social immunity