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

Robert Gauthier/Los Angeles Times / MCT

Health officials now say 32 people tied to a December Disneyland trip have come down with the measles, up from the initial estimate of nine.

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.

Twenty-eight of the cases are in California, two in Utah, one is in Colorado, and one in Washington state, the Los Angeles Times reported. Most spent time in the parks in December, though a few may have caught the illness from others who visited.

Doctors in San Diego were also testing five people who arrived at an urgent care center Wednesday with measles symptoms. The clinic closed temporarily as health workers attempted to limit any further potential exposure to the virus.

In Colorado, health officials said a measles patient was treated at Penrose Hospital in Colorado Springs and may have exposed more people to the virus. Another woman traveled from Orange County to Washington via Sea-Tac Airport, Washington health officials said. She also spent time in other public places, including shops and an emergency room, before flying home to Orange County.

The patients range in age from infants to adults, officials said. Some were partially vaccinated and at least two were too young to have been administered the vaccine, 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 typically 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.