PHOENIX -- A measles outbreak in Arizona that originated at California's Disney parks is at risk of increasing dramatically in size as health officials keep tabs on 1,000 people, including nearly 200 children who could have been exposed at a Phoenix-area medical center.

Those who haven't been vaccinated are being asked to stay home for 21 days, a standard health practice, or wear masks if they have to go out in public. State Health Services director Will Humble said it's possible but unlikely that the number of cases can be contained at seven.

"To stay in your house for 21 days is hard," he said. "But we need people to follow those recommendations, because all it takes is a quick trip to the Costco before you're ill and, 'bam,' you've just exposed a few hundred people. We're at a real critical juncture with the outbreak."

At least 95 cases of measles have been linked to the outbreak traced to Disney theme parks, according to the California Department of Public Health. Arizona has the second most, after California, with measles confirmed in six other states -- Michigan, Utah, Washington, Colorado, Oregon and Nebraska -- and Mexico. Most of those infected were not vaccinated, and health officials have urged people to get the measles shot.

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Health officials don't know yet how many of the Arizona children being monitored were vaccinated, or their age ranges. Children under a year old cannot receive the vaccination for measles, mumps and rubella but can get an immunity booster. Health officials were working to notify the families of children who visited the Phoenix Children's East Valley Center from Jan. 20-21.

The Arizona woman whose case was confirmed Tuesday in Maricopa County came into contact with a Pinal County family that traveled to Disneyland, but did not have telltale signs of measles like a rash when she visited the Phoenix Children's East Valley Center. Maricopa County health director Bob England declined to say whether she'd had the measles vaccine, which isn't 100 percent effective in stemming the spread of the disease.

"Unfortunately, she came down with the disease and by the time it was recognized had already exposed a large number of children at the facility," he said.

England said health officials will check in with the families of the children following the 21-day incubation period, which ends Feb. 11-12. He said most parents understand the importance of keeping their children home.

Phoenix Children's spokeswoman Debra Stevens said Wednesday that anyone who suspects they have measles is being asked to call ahead so that staff can take the necessary precautions to help keep measles from spreading, including bringing masks to incoming patients.

"If someone has chosen not to vaccinate their children or for some reason cannot vaccinate their children, they face a higher responsibility now to let their health care provider know in advance," she said.

Meanwhile, masks are being placed outside health care facilities and signs went up outside a handful of places in Kearny warning customers and employees that they could have been exposed to measles. A man who is recovering from measles also came into contact with the Pinal County family that visited Disneyland.

Gila County health officials say they are tracking 17 people who were at a hospital in Globe in the same time frame as a person confirmed to have measles.