The number of people sickened by the highly contagious, occasionally deadly disease climbed by 78 during the first week of April, as four more states reported their first cases of 2019. Now, measles has been found in more than a third of U.S. states — up and down both coasts, and across the plains, the Midwest and the South — with most of the illnesses occurring in children.

In 2000, officials announced that they had eradicated measles in the United States. Yet since then, there have been years when the number of cases has surged, notably in 2014, when 667 were reported — the highest annual total since the turn of the century. That year, the disease was reported at a rate of 1.83 cases per day. In 2019, however, the rate has increased to 4.84 cases a day. If that pace continues, this year could surpass 2014 by June.