(CNN) -- The number of measles cases in the United States so far this year has risen to 288, the highest number for one year since at least 2001, 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.
CNN's Elizabeth Cohen contributed to this report
™ & © 2014 Cable News Network, Inc., a Time Warner Company. All rights reserved.