The existence and spread of a disease can be warranted by just a tiny number of unvaccinated people.

To control the spread of a disease, the scientific community introduced the concept of "herd immunity," which recommends the vaccination of a large population, be it for flu or measles. It is believed that vaccinated people become obstructions in cases of outbreaks, as diseases can’t be transmitted through them to other people.

In this way, the most susceptible population; infants below 12 months of age in whom vaccines are contraindicated, and elderly individuals in whom these vaccine-preventable infections could prove fatal, and immuno-compromised persons who can’t be vaccinated and who could die due to these infections, are protected by these obstructions.

Different diseases have different thresholds for herd immunity as the pathogen and mode of transmission differ in each. An analysis of different vaccine-preventable illnesses by the Centers for Disease Control and Prevention marked the lowermost threshold of the number of people to be vaccinated at 75 percent for mumps and the uppermost at 94 percent for measles. Even the least threshold necessitates vaccination of at least three in four people.

To achieve eradication of diseases in the long run, especially the ones exclusive to humans, the public health aspires to vaccinate everyone possible by attaining herd immunity for maximum vaccine-preventable diseases.