It may be the happiest place on Earth, but California health officials are warning people to stay away from Disneyland unless they’re vaccinated.

That means all babies under the age of 1, who are too young to get the vaccine, and people who for various other reasons have not been immunized or who haven’t already had measles.

“People ask whether it is safe to visit venues where measles has been identified and may be circulating. The answer is yes, considering you have been fully vaccinated,” Dr. Gil Chavez, state epidemiologist for California, told reporters. “Parents of infants may consider staying away.”

The highly contagious and potentially deadly virus has already infected at least 70 people in six states and in Mexico. And given that people from all over the world visit Disneyland, it’s almost certain there will be many more cases in more places. The latest new case: an Arizona woman in her 50s who visited the theme park in December.

“Parents of infants may consider staying away.”

“We have had, in two and a half weeks, as many cases as we had in all of last year (in California),” Chavez said. “From this particular outbreak, we can expect to see additional cases.”

Infectious disease experts say to expect many more cases. Measles is one of the most infectious viruses known. It spreads through the air and can linger in a room after the sick person has left. People can spread it before they even have symptoms, and measles may look like flu in some people before they develop the characteristic red rash.

Measles is more contagious than polio, smallpox or flu. In an unvaccinated population, each measles patient can be expected to infect 11 to 18 others. The only infection that comes close is whooping cough. Compare that to the worst pandemic influenza, in which one patient infects two or three others directly, or the AIDS virus, in which one patient can infect on average two to five others.

“People who are unvaccinated should know that there is presently a risk for getting measles in California,” Chavez said. He’s urging anyone who is not vaccinated to get the shot now.

People who have had measles or who have been vaccinated generally aren’t at risk. But 90 percent of unvaccinated people with no immunity will catch measles if they breathe it in or come in contact with it in some other way.

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And measles has a 21-day incubation period, which means people who have been infected can travel far and wide before they get sick and alert a doctor.

“We can expect to see many cases of this vaccine-preventable disease unless people take precautionary measures,” Chavez said.

State and local disease detectives will be tracking down everyone who’s been in contact with each and every case that is reported. “This could easily amount to hundreds of contacts for each case,” said Kathleen Harriman, chief of the Vaccine Preventable Disease Epidemiology Section at California’s Department of Public Health.

“We want to identify and isolate these cases very early on,” Chavez added.

Five park employees are among the cases, and Disney is watching about 100 other employees who worked closely with them.

Measles has been just about eliminated in the United States by vaccination. Before the vaccine was available, measles made about 500,000 people sick in an average year, and it killed around 500 of them. Pneumonia is what kills most people who die but encephalitis, an inflammation of the brain, also can develop and kill patients.

“I want to stress at this time that measles is not a trivial illness,” Chavez said. “Those of us who have cared for measles patients can attest to its potential severity.”

Most U.S. outbreaks are linked to travel. Visitors to the U.S. do not have to show they’ve been vaccinated against measles, and unvaccinated Americans can travel abroad and bring the virus back with them. They can then infect others who haven’t been vaccinated – including babies and people who refuse or cannot get the vaccine.

And as many as 5 percent of people who get vaccinated don’t develop the full immune response needed. And immunity may have waned in others many years after vaccination.

Some people cannot be vaccinated because of allergies or because of legitimate religious objections. And babies too young to be vaccinated are very vulnerable. So doctors aim to create what’s called herd immunity – if everyone who can be vaccinated gets vaccinated, the virus is far less likely to spread and infect those who are not.

"Measles is not a trivial illness.”

What’s helping fuel the regular U.S. outbreaks now are clusters of people who for other reasons have decided not be vaccinate themselves or their children. The virus can take hold and spread from these clusters.

“Vaccination is safe and effective,” Chavez said. Doctors stress that there are almost no health concerns about the combined measles, mumps and rubella (MMR) vaccine. Study after study has proven the MMR vaccine does not cause autism, and there is no advantage to “spacing out” the vaccine over time. Kids should get two doses at 12 months and 15 months and adults who are not immune or not vaccinated should also get two doses spaces three months apart.

The vaccine can be given to people who have already been exposed to measles and it may prevent infections from taking hold, but it takes about two weeks to provide full immunity.

“It is really a bad combination when you have people that trivialize what can be a very dangerous illness and then, on bad information, decide that vaccines are unsafe,” Chavez said.

The Associated Press contributed to this article