Vaccination 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 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 ( 'cow'). Smallpox was a contagious and deadly disease, causing the deaths of 20–60% of
Vaccination policy once vaccination rates reach a certain level, even if this level is below that optimal for the community. For example, a 2003 study found that a bioterrorist attack using smallpox would result in conditions where voluntary vaccination would be unlikely to reach the optimum level for the U.S. as a whole, and a 2007 study found that severe influenza epidemics cannot be prevented by voluntary vaccination without offering certain incentives. Governments often allow exemptions to mandatory vaccinations for religious or philosophical reasons, but decreased rates of vaccination may cause loss of herd immunity, substantially increasing risks even to vaccinated individuals.