"Health and medical scholars have described vaccination as one of the top ten achievements of public health in the 20th century. Yet, opposition to vaccination has existed as long as vaccination itself. Critics of vaccination have taken various positions, including opposition to the smallpox vaccine in England and the United States in the mid to late 1800s, and the resulting anti-vaccination leagues; as well as more recent vaccination controversies, such as those surrounding the safety and efficacy of the diphtheria, tetanus, and pertussis (DTP) immunization, the measles, mumps, and rubella (MMR) vaccine, and the use of a mercury-containing preservative called thimerosal."