In 2000 america declared measles “eliminated”, meaning that the virus was no longer indigenous and any new infections were linked to strains brought in from abroad. In the following decade measles in America remained rare. Now cases are on the rise again. There were 372 in 2018, the second highest number since 1996. Over 200 were reported in the first two months of this year. Though the disease is rarely deadly, it often requires hospitalisation.

Most recent large outbreaks have been in insular religious or immigrant groups, such as the Amish, Orthodox Jews and Somali-Americans. Some had been lectured or leafleted by crackpots who claim that vaccines are harmful. They are easy prey for such conspiracy theories because language and cultural barriers keep them at a distance from mainstream health care. Low vaccination rates have made them hotspots for outbreaks, often ignited by measles picked up on visits to relatives in countries where the disease is widespread.