Merriam-Webster defines a vaccine as "a substance that is usually injected into a person or animal to protect against a particular disease". Vaccines are generally weakened or inert versions of a disease that you are given to jumpstart the body's immune system against the disease so that when exposed to the full version of the virus, the vaccinated individual will not get the disease. Vaccines arose from innoculations which came to the West in the early 18th century. Innoculation had been practiced in the East for centuries.
Vaccines have been controversial for almost as long as there have been vaccines. Edward Jenner is credited with creating the first vaccine in 1796; The Anti-Vaccination Society was founded in 1798 followed by the London Society of Abolition of Compulsory Vaccination in 1800.
Vaccine controversies come and go.
The curent controversy, the idea that vaccines, including their components and schedules, can lead to autism and other diseases started in the 1980s and 1990s. It was greatly bolstered by the 1998 Lancet paper by Andrew Wakefield that linked the measles, mumps and rubella vaccine with autism. While his paper were shown to be fraudulent, they are still cited by those who believe in the connection.