Herd immunity, otherwise known as community immunity, occurs when a large population of an area develops resistance to or protection against certain diseases. When sufficient people become immune to the cause, be it bacteria or virus, it helps cut down on the spread of the ailment. Though some may still be susceptible to that contagious disease, the whole group is considered resistant to it as there are a decreased number of at-high-risk people.

Importance of Herd Immunity
Herd immunity is important in protecting at-risk populations, as the disease gradually fades out and infection rates fall due to this phenomenon. This helps safeguard those with weak immune systems, and babies, who are not yet disease-resistant independently. Herd immunity is achieved by exposure to prior illnesses or through vaccinations, depending on the disease. However, in terms of vaccine protection, some demographic sections might not be able to get them and remain at risk. These include pregnant women, babies, and immunocompromised individuals, like those undergoing or having gone through organ transplants or chemotherapy. Through herd immunity, these people can also be protected. When enough people in a population have gotten vaccinated against the deadly diseases, the germ lacks any potency and the population is safe.

Evolutionary Role
Herd immunity, on its own, also has an evolutionary role to play. It has a certain evolutionary pressure on pathogens that influences viral evolution by encouraging the production of novel strains known as escape mutants. They can escape herd immunity and can gain the potential to infect the previously immune populations. This evolution brought about through herd immunity is called serotype shifting or serotype replacement. This is because the dominance of specific serotypes becomes lower due to high immunity levels that allow other types to take their place. At a molecular level, viruses can evade herd immunity through the accumulation of mutations in the viral genome encoded for its surface antigen. This antigen is generally a protein that causes changes in the viral epitope.

Threshold
Herd immunity is considered when a large chunk of a particular population, or the herd, becomes immune to a particular disease. This helps to keep infectious germs from spreading from person to person. As a result, the entire community is considered safe, and not just those who are immune due to vaccination or other factors. An important aspect of understanding herd immunity is to know what a threshold population is for a certain ailment. This refers to the specific percentage of individuals who should be immune to the condition, typically due to a vaccine. If the resistant population that cannot get the disease is higher in number than that of the remaining population, the pathogen’s spread will get slower. This state is referred to as the herd immunity threshold. The percentage of people who need to be immune varies depending on various factors of the diseases, including the ability to be transmitted, how contagious the condition is, etc. If the disease is highly communicable, higher the threshold to achieve herd immunity, or the portion of the population that should remain immune to the disease to stop it from spreading. Examples of varying herd thresholds include 80% population for polio and 95% for measles due to its higher infection potential.