THE measles vaccine is a victim of its own success.

In 1963, when the vaccine first came into existence, measles virus infected about three million people a year in the United States, hospitalized 48,000 and killed 500. By the turn of the 21st century, however, measles infections had been virtually eliminated. Unfortunately, because some parents are choosing not to vaccinate their children, measles is coming back. Last year, nearly 200 cases were reported. This year, the numbers appear to be even worse, with cases in more than half a dozen states already.

The current outbreak in New York City revealed another problem. The city’s deputy commissioner for disease control, Jay Varma, found that some of the cases might have resulted from exposure in hospitals. On March 12, one hospital wrote an email to the staff stating that “nearly 600 patients” might have been exposed. Given its rigorous infection-control procedures, a hospital would seem to have measures in place to prevent such exposures. But the lapse shouldn’t be so surprising.

First, most clinicians and parents have never seen someone with measles. When potential cases come into emergency departments or outpatient clinics, physicians often call in older colleagues to determine whether a child is infected with measles or with other more common viruses that cause fever and rash. Some of these patients aren’t isolated immediately, and wait among other people, or walk the halls. That leads to the next problem.

Measles is remarkably contagious. Probably the best example is a measles outbreak that occurred in Indiana in 2005 after an unimmunized teenager traveled to Romania as part of a church group, and visited a hospital and orphanage. When she came home, she didn’t know she was infected. The next day, at the beginning of her illness — when she was most contagious — she attended a church gathering of about 500 people. Of the approximately 450 who had evidence of immunity, less than 1 percent came down with measles. Of the remaining 50 who did not, 32 percent became infected. Among the 35 church members whose parents had declined immunization, 31 eventually caught the virus — 16 of whom were infected that day. They didn’t have to have face-to-face contact with the girl; all they had to do was inhabit her airspace within two hours of her having been there.