(CNN) -- The number of measles cases in the United States this year has risen to 288, the highest number for one year since the disease was eliminated from the country in 2000, the Centers for Disease Control and Prevention said Thursday.
This also is the largest number of measles cases that the country reported in the first five months of a year since 1994, according to the CDC.
"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.
Serious complications and death may result from measles, which is highly contagious.
Elimination means there is no continuous disease transmission for at least 12 months in a specific geographic area. Measles is not native to the United States anymore, but cases may arise as people bring the disease into the country from abroad.