Last Updated Jan 22, 2015 6:37 PM EST
LOS ANGELES -- At least 75 people have been infected in a measles outbreak that led California public health officials to urge those who haven't been vaccinated against the disease, including children too young to be immunized, to avoid Disney theme parks, where the spread originated.
New infections linked to the theme parks continue to emerge, and the outbreak has now spread to six U.S. states and Mexico. On Thursday, Arizona health officials confirmed the first case in that state, a 50-year-old woman from the Phoenix area who visited Disneyland in mid-December. She has since recovered.
The vast majority of cases have occurred in California.
Because measles is highly contagious, people who have not received the measles-mumps-rubella, or MMR, vaccine are susceptible and should avoid visiting Disney "for the time being," state epidemiologist Gil Chavez said.
The same holds true for crowded places with a high concentration of international travelers, such as airports, Chavez said. People who are vaccinated don't need to take such precautions, he said.
Disneyland Resorts spokeswoman Suzi Brown said officials agreed with the advice that "it's absolutely safe to visit if you're vaccinated."
The people who have been infected range in age from 7 months to 70 years old. The vast majority had not been vaccinated, and a quarter had to be hospitalized.
Among those sickened were five Disney employees. Three have since returned to work. The company previously said park employees who may have been in contact with those infected were asked to show proof of vaccination or have a blood test to show immunity against measles. Those with pending results were put on paid leave. Vaccinations are also being offered to all employees.
Measles has hit California hard recently. The state typically sees four to 60 measles cases a year, and has already topped that in the first three weeks of 2015.
"We are off to a bad start in 2015," Chavez said.
Since the outbreak, two dozen unvaccinated students at an Orange County high school were sent home for three weeks after an infected pupil showed up.
Measles is a highly contagious respiratory illness that can spread by air through coughing or sneezing. Symptoms include fever followed by cough, runny nose and a blotchy rash.
The virus apparently can hang in the air for up to two hours after an infected person has coughed, officials say, notes CBS Los Angeles. Measles symptoms show up between 10-12 days after exposure and could show up as much as 21 days later.
Though the virus has been eradicated in the U.S. since 2000, it can still enter the country through an infected traveler.
While health officials said they likely may never find "patient zero" or the trigger of the outbreak, they believe it was either a resident from a country where measles is widespread or a Californian who went abroad and brought home the virus.
People at highest risk are those who are unvaccinated, pregnant women, infants under 6 months old, and those with weakened immune systems.