CDC director Tom Frieden discusses his “personal enemy,” the measles, which he says is probably the most contagious of all infectious diseases.

Measles is a personal enemy of mine. My first assignment at the Centers for Disease Control and Prevention as a disease detective 25 years ago was to investigate a large measles outbreak in New York City. Many of the cases were children under a year old. Nationally, there were more than 55,000 cases, and more than 100 children died. By increasing vaccinations, we ended the New York outbreak – and the national outbreak was ended by a push to vaccinate every child by age 2.

It’s only February, and already we’ve had 141 measles cases in 17 states and Washington, D.C. At least 80% of the patients have been part of a large, ongoing outbreak linked to Disneyland. It’s not clear how this outbreak began, although most likely an unvaccinated person brought measles to the United States after international travel.