The measles outbreak that originated at a California theme park continues to grow, causing increased concern among public-health authorities.
The large number of cases so far this year has already eclipsed the number typically reported each year, according to Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases.
"This worries me and I want to do everything possible to prevent measles from getting a foothold in the United States and becoming endemic again," Schuchat said during a briefing with reporters on Thursday.
Measles was deemed to be eliminated in the United States in 2000, but last year saw an incredible resurgence for the virus. There were 644 cases last year, more cases than occurred in the previous five years combined.
Most of the people who contract measles have not been vaccinated, according to the CDC.
Officials believe the outbreak is linked to someone with measles visiting Disney theme parks in California in December.
That state has been hit the hardest by this outbreak, with the California Department of Public Health reporting 91 cases there so far this year. In Arizona, meanwhile, more than 1,000 people -- a fifth of them children -- are being monitored for potential exposure.