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.”

Health officials don’t know yet how many of the children 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.

Arizona is second to California in the number of cases traced to visits to Disney parks last month. Measles has been confirmed in five other states – Utah, Washington, Colorado, Oregon and Nebraska – and Mexico. Most of those infected were not vaccinated.