The Centers for Disease Control and Prevention announced Thursday that so far in 2014, there have been 307 cases of the measles, the most recorded since 1996.
"Ninety percent of all measles cases in the United States were in people who were not vaccinated or whose vaccination status was unknown," the CDC said in a news release.
The 288 cases were reported in 18 states from January 1 to May 23, the CDC said.
The CDC says timely vaccination is the best way to prevent measles.
Measles was eliminated from the United States in 2000, meaning that for more than 12 months there was no longer any continuous measles transmission.
"The current increase in measles cases is being driven by unvaccinated people, primarily U.S. residents, who got measles in other countries, brought the virus back to the United States and spread [it] to others in communities where many people are not vaccinated," Assistant Surgeon General Dr. Anne Schuchat said in a U.S. Centers for Disease Control and Prevention news release.
"Many of the clusters in the U.S. began following travel to the Philippines, where a large outbreak has been occurring since October 2013," added Schuchat, who's also director of CDC's National Center for Immunization and Respiratory Diseases.
The initial symptoms of measles develop around 10 days after infection. These can include cold-like symptoms, red eyes and sensitivity to light, a high temperature or fever and greyish white spots in the mouth and throat.
Measles is still common in many parts of the world, including countries in Europe, Asia, the Pacific, and Africa.
