Up to 1,000 people, including 200 children, are being monitored for possible exposure to measles at Phoenix Children's East Valley Center, officials said on Wednesday.
Arizona officials said that despite the high number, people who were vaccinated are not likely in any danger.
The outbreak originated in California's Disneyland parks, which has now infected at least 65 people.
There have been seven confirmed cases in Arizona. The highly contagious virus has also been confirmed in five other states — Utah, Washington, Colorado, Oregon and Nebraska — as well as in Mexico.
Arizona officials said they do not know exactly how many people were exposed in the medical center because it's unclear how many of the children were vaccinated. Kids under one year old cannot receive measles vaccinations, but may be given an immunity booster.
Officials said they were in the process of notifying parents of children who had visited the medical center on Jan. 20 or 21.
A woman is believed to have caught the virus when she came into contact with a Pinal County, Arizona, family that had recently returned from a trip to Disneyland.
When the woman visited the Phoenix Children's East Valley Center last week, she did not have the most obvious signs of the virus, such as a rash, and it was not confirmed she had the measles until Tuesday.
"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," Maricopa County health director Bob England said.
Those who haven't been vaccinated are asked to stay home for 21 days, a standard quarantine procedure, or wear masks if they go out.
"To stay in your house for 21 days is hard," State Health Services director Will Humble 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 said they will check in with the families after the 21-day period, which ends Feb. 11-12.