Health officials said Monday that 704 people have now been infected with the highly infectious measles virus, the most in 25 years.

Dr. Robert Redfield, director of the Centers for Disease Control and Prevention, told reporters in a phone call Monday that most infections were among children who had not been vaccinated, and other officials warned that people opposed to vaccinations were spreading misleading information about the vaccine that prevents measles to dissuade parents from getting their children vaccinated.