(CBS) - The number of measles cases in the United States reported so far this year has been the highest in the past 20 years, according to a new report from the Centers for Disease Control and Prevention (CDC).
The CDC received reports of 288 confirmed measles cases in the United States between January 1 and May 23, 2014.
Measles was declared eliminated in this country in 2000, so why is it spreading anew? Experts say most U.S. outbreaks can be traced to unvaccinated people who traveled internationally to parts of the world where the disease is still common. Of all the cases reported to the CDC, 280 (97 percent) were tracked back to at least 18 countries.
"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 to others in communities where many people are not vaccinated," Dr. Anne Schuchat, an assistant surgeon general and director of CDC's National Center for Immunizations and Respiratory Diseases, said in a statement. "Many of the clusters in the U.S. began following travel to the Philippines where a large outbreak has been occurring since October 2013."
More than one in seven cases of measles led to hospitalization, according to the CDC.
Ninety percent of all measles cases in the United States occurred in people who were not vaccinated or whose vaccination status was unclear. The CDC also reported that most of them had refused vaccination for religious, philosophical or personal reasons.
The largest measles outbreak this year -- 138 confirmed cases so far -- occurred in central Ohio, where it spread among the Amish community.
"The Amish population in Knox County has been very cooperative in getting vaccinated or self-reporting (if they have symptoms) or staying home if they do get the measles," county health commissioner Julie Miller said in a statement. "Yet, there's a good chance that someone who doesn't know they have been exposed will pass the virus on to the non-Amish population - who we commonly refer to as the English."
Measles is a serious and highly contagious respiratory disease. Vaccination is the best way to prevent measles, the report emphasized. The vaccine is normally given in two doses, at the ages of 12-15 months and 4-6 years.
Despite this year's rising numbers, the disease is still extremely rare in the U.S. Doctors may therefore have trouble recognizing symptoms of measles, which include fever, rash, cough, runny nose and pink eye.
"Many U.S. health care providers have never seen or treated a patient with measles because of the nation's robust vaccination efforts and our rapid response to outbreaks," Schuchat said.