The rising number of measles cases this year is by far the most since successful vaccination led to elimination of the disease in this country almost two decades ago. Cases appear in the United States every year because of people traveling abroad or visitors bringing measles from other countries where it is still common. Lower rates of vaccination, however, have created increased risk when the virus is brought in from other countries.

At least 764 cases across 23 states have been reported this year, according to the Centers for Disease Control and Prevention.

Two overlapping groups have suffered the most this year: children under age 4 and people who were unvaccinated.