The airborne, highly infectious virus is best stopped by getting vaccinated, officials said.

Robert Gauthier/Los Angeles Times / MCT

Health officials believe one person already sick with the highly contagious virus went to Disneyland or the neighboring California Adventure between Dec. 15 and 20, infecting visitors.

Sixteen of the cases were in California, two in Utah, and one is in Colorado, the Associated Press reported.

The patients range in age from 8 months to 21 years old, officials said. Some were partially vaccinated and at least two were too young to be vaccinated, according to the AP.

Measles has been essentially eliminated in the U.S. since 2000, but in Western Europe, large outbreaks have occurred amid declining vaccination rates.

"Measles vaccination is important because before the introduction of measles vaccine in the United States in 1963, there were approximately 500,000 cases and 500 deaths a year," said Carlos Villatoro of the California Department of Public Health, adding that 90% to 95% of the population must be immune to measles to sustain the virus' elimination.

The virus travels through the air, Villatoro said, and where it can linger for up to two hours and travel long distances on air currents. In one measles outbreak, the virus spread from one person in a sports arena to another sitting 100 feet away, Villatoro said.

Patients on average develop a rash two weeks after being exposed to the virus. They are contagious four days before the rash develops and four days after.

Before the rash develops, symptoms include runny nose, high fever, red eyes and a cough.

Almost all U.S. measles cases can be traced to a traveler who spent time in countries where measles is prevalent, officials said. With Disneyland's worldwide popularity, officials said that's probably what happened this time as well.

"We have not identified the source case for this outbreak, but it is likely to have been an international visitor," Villatoro said.