For decades, measles existed mostly as a memory in the United States, a disease largely consigned to textbooks and history lessons. After the measles vaccine was introduced in the 1960s, cases decreased from millions each year to near elimination. Parents could stop fearing the fever and rash that once defined a pervasive, sometimes deadly childhood illness. But in recent years, measles has returned in the U.S., with a 25-year high last year of 2,144 cases. As of Friday, 1,136 confirmed cases have been reported so far this year. According to experts, including Dr. Nicole M. Iovine, an infectious disease physician and hospital epidemiologist at the University of Florida , the resurgence is driven by a simple and dangerous reality: more people are no longer immune. As vaccination rates decline or families delay routine immunizations, even small pockets of vulnerability can allow the virus to spread rapidly. “You only have to open the door a little bit,” said Dr. Iovine.…