Michael Specter reports in The New Yorker that the Washington State Department of Health has announced the first confirmed measles death in the United States in more than a decade, and explains that the “herd immunity” gained by vaccinations is lost when the vaccination rate falls below ninety person — as is the case in Washington state.

One of the central purposes of universal vaccination is to provide “herd immunity” to the most vulnerable segments of the population—infants, for example, and those receiving drug treatment for cancer and other diseases that compromise the immune system. If you are sick and unable to get vaccinated, the herd around you, in theory, should provide protection. Once the vaccine rate falls much below ninety per cent, however, herd immunity disappears. Vaccine rates are particularly low in northwestern Washington, where the measles death occurred, as they are in many other parts of the state.*

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