Anthrax vaccines known to result in far less scarring, and greatly reduced mortality, in comparison with the naturally acquired disease. The English physician Edward Jenner (1749–1823) had also discovered (1796) the process of vaccination by using cowpox to give cross-immunity to smallpox and by Pasteur's time this had generally replaced the use of actual smallpox material in inoculation. The difference between smallpox vaccination and anthrax or chicken cholera vaccination was that the weakened form of the latter two disease organisms had been "generated artificially", so a naturally weak form of the disease organism did not need to be found. This discovery revolutionized
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.