Vaccination is the administration of a vaccine to help the immune system develop protection from a disease. Vaccines contain a microorganism or virus in a weakened or killed state, or proteins or toxins from the organism. In stimulating the body’s adaptive immunity, they help prevent sickness from an infectious disease. When a sufficiently large percentage of a population has been vaccinated, herd immunity results. The effectiveness of vaccination has been widely studied and verified  Vaccination is the most effective method of preventing infectious diseases; widespread immunity due to vaccination is largely responsible for the worldwide eradication of smallpox and the elimination of diseases such as polio and tetanus from much of the world.

Smallpox was most likely the first disease people tried to prevent by inoculation and was the first disease for which a vaccine was produced The smallpox vaccine was invented in 1796 by English physician Edward Jenner and, although at least six people (the case of Thomas must be considered and regarded as a remarkable predecessor) had used the same principles years earlier, he was the first to publish evidence that it was effective and to provide advice on its production. Louis Pasteur furthered the concept through his work in microbiology. The immunization was called vaccination because it was derived from a virus affecting cows (Latin: vacca‘cow’). Smallpox was a contagious and deadly disease, causing the deaths of 20–60% of infected adults and over 80% of infected children. When smallpox was finally eradicated in 1979, it had already killed an estimated 300–500 million people in the 20th century.

Vaccination and immunization have a similar meaning in everyday language. This is distinct from inoculation, which uses unweakened live pathogens. Vaccination efforts have been met with some controversy on scientific, ethical, political, medical safety, and religious grounds, although no major religions oppose vaccination, and some consider it an obligation due to the potential to save lives. In the United States, people may receive compensation for those injuries under the National Vaccine Injury Compensation Program. Early success brought widespread acceptance, and mass vaccination campaigns have greatly reduced the incidence of many diseases in numerous geographic regions