In California, it was the perfect breeding ground for a public health crisis: a theme park. Disneyland not only attracted millions every year from all over the globe, but many of the children who descended on the popular resort in late 2014 were not vaccinated against measles, one of the world’s most contagious diseases. Within weeks, the illness raged across the state — with many hospitalized — in the largest outbreak of the disease in California in two decades.