Reddit reviews The History of Vaccines

Variolation (or innoculation) was a technique developed ~1000 C.E. in Asia. At some point, someone recognized that if you were infected with smallpox once, people usually didn't get it again. So variolation intentionally infected an individual with smallpox. This lead to milder smallpox infection, but as you might imagine, this still had some major problems. Variolation killed people, but it was far safer than natural infection, and more controlled than a natural outbreak. Still, it was highly controversial for many different reasons. Some people did not think the benefit outweighed the risk, and others opposed variolation on religious grounds. In the late 18th Century, some American colonies had outlawed variolation (and once these laws were lifted many people rushed to become innoculated). We see this in Puritan New England at the time, and as a result, Boston had a serious epidemic of smallpox during the Revolutionary War.

In 1798, Edward Jenner, an English physician, published his work in discovering vaccination. Jenner discovered that a person infected with cowpox (now thought to be a very weak mutation of smallpox), would be protected from smallpox. Jenner's technique had far fewer side-effects than variolation, and produced excellent immunity. However, the resistance was almost immediate.

> Widespread smallpox vaccination began in the early 1800s, following Edward Jenner’s cowpox experiments...

> Some objectors, including the local clergy, believed that the vaccine was “unchristian” because it came from an animal. For other anti-vaccinators, their discontent with the smallpox vaccine reflected their general distrust in medicine and in Jenner’s ideas about disease spread. Suspicious of the vaccine’s efficacy, some skeptics alleged that smallpox resulted from decaying matter in the atmosphere. Lastly, many people objected to vaccination because they believed it violated their personal liberty, a tension that worsened as the government developed mandatory vaccine policies.

To be fair, this was a time before physicians understood that germs caused disease, and there were still some unintended consequences of vaccination. If a physician used an unclean needle for administering smallpox vaccination, he could unintentionally infect people with other diseases (tetanus, hepatitis...). Still, the reasonable concerns of the anti-vaccine movement were largely overshadowed by others claiming that cowpox would literally turn you into a cow.

There is more information from the College of Physicians of Philadelphia's History of Vaccines website.

EDIT: Spelling. Also if you are interested in more in-depth history of vaccines and the anti-vaccine movement, I suggest the following books:

• The History of Vaccines
  
• Vaccine: The Controversial Story of Medicine's Greatest Lifesaver
  
• Deadly Choices: How the Anti-Vaccine Movement Threatens Us All