evidence: The Amish are a Christian Anabaptist group of Swiss-German ancestry who arrived in the U.S. in the 18th century. Today about 335,000 Amish people live in 31 U.S. states. The Amish religion does not restrict access to modern medical care and children are vaccinated to some degree. Independently of immunization status and lifestyle, the Amish population has less genetic variability, which may be protective against certain diseases while increasing susceptibility to others.