Largely because of resistance to vaccination, cases of measles have reached a 20-year high in the United States, federal health officials said on Thursday.

As of May 23, there were 288 confirmed cases in the United States — more than in all of 2013, and more than in the equivalent period of any year since 1994. The number is expected to increase during the summer travel season.

“This is not the kind of record we want to break,” said Dr. Anne Schuchat, director of immunization and respiratory diseases for the Centers for Disease Control and Prevention.

The measles virus, which is highly contagious, usually causes only a fever and a rash. But it can lead to pneumonia, brain damage, deafness and even death. An unvaccinated American child who develops measles has about a one in 500 chance of dying, even with hospital care, according to the C.D.C.