Measles is among the most contagious of diseases. Virus-laced droplets can hover in still indoor air for up to two hours after someone infected has coughed or sneezed. Up to 90 percent of people who are exposed will catch the virus if they are not immunized.

The vaccine is considered very safe, and two doses are about 97 percent effective at conferring immunity. The vaccine is normally given at ages 1 and 5, but during outbreaks pediatricians may give it to healthy children as young as six months old.

Around the world, measles cases fell 80 percent between 2000 and 2016, with deaths dropping to 90,000 a year from 550,000. But two years ago, cases began rebounding, driven by a combination of poverty, warfare, tight vaccine supplies and, in some countries, hesitation about vaccination.

Earlier this month, the W.H.O. said there were three times as many measles cases around the world this year as there were in the first three months of 2018.

Outbreaks of tens of thousands of cases have occurred recently in poor or war-torn countries like Madagascar, Ukraine and Yemen. But case numbers are also climbing in wealthy countries with modern health care systems, like Israel, Britain, France and Italy. Deaths from measles have occurred in those countries.

Before measles vaccination became widespread in the United States in 1963, up to four million Americans got measles each year, the C.D.C. said. Of the roughly 500,000 cases that were reported to medical authorities, about 48,000 were hospitalized, 4,000 developed encephalitis, and 400 to 500 died.

Nationally, since the mid-1990s, more than 91 percent of American children have been vaccinated against measles. (Anyone born before 1957 is assumed to have had the disease as a child and to be immune to it.)