The case was diagnosed among evacuees at the Miramar Marine base in San Diego. A second person is being evaluated for possibly having the virus.
"Both patients are doing well and have minimal symptoms," according to the UC San Diego Health system.
In the meantime, 195 evacuees who were brought to March Reserve Air Base in Riverside are scheduled to be released at the end of their 14-day quarantine on Tuesday. There were no known cases of coronavirus among that group.
Riverside County health officials urged the public to treat evacuees and employees of the base with respect. They emphasized that base employees were not in physical contact with evacuees during the quarantine period.
Dr. Cameron Kaiser, public health officer for Riverside County wrote to the public:
"We have heard your concerns about the evacuees on the base. Many of these concerns are reasonable. Unfortunately, some people with these concerns have taken them out on the families and households of people working at March Air Reserve Base. There have been comments made that have been hurtful - both in person and on social media - that are often based on incorrect or incomplete information. A few base workers have even been accosted in uniform. This is not acceptable, and needs to stop."
"Please understand that people on and off the base are not at increased risk for exposure to the new virus, and we don't restrict people who don't actually pose any risk. They have had no contact with the evacuees, whose area is fully separated from base personnel. You do not need to exclude household or family members of MARB personnel, nor do you need to require them to obtain unnecessary 'clearance letters' from a physician or health authority. They pose no greater risk than anyone else."
The Miramar group at first was also thought to be free of coronavirus. Four individuals had been isolated in a hospital after showing symptoms, but on Sunday federal health officials said they had tested negative and they were sent back to the base.
On Monday morning, however, officials with the U.S. Centers for Disease Control and Prevention informed county health officials that "further testing revealed that one of the four patients tested positive" for novel coronavirus and the person was returned to hospital isolation, UC San Diego Health said.
This is the seventh confirmed case of novel coronavirus in California and the 13th in the United States.
The person confirmed to have the illness arrived at Miramar last Wednesday from Wuhan. That locked-down city of 11 million is the epicenter of the highly contagious disease, which has killed more than 1,000 people overseas.
Recent chartered flights have sent hundreds of people back to the U.S. They are being held in quarantine at military bases in California, Texas and Nebraska.
Five evacuees taken to Travis Air Force Base, located between San Francisco and Sacramento, were hospitalized after showing symptoms of the virus but none of those possible cases has been confirmed, authorities said.
No symptoms were reported among evacuees at Lackland Air Force Base in San Antonio or a Nebraska national Guard training base in Omaha.
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Worldwide, there are believed to be at least 43,000 cases of coronavirus infection. In China, where the outbreak is centered around Wuhan, officials announced the death toll had recently passed 1,000.
The outbreak has resulted in the evacuations of hundred of U.S. citizens on State Department-chartered flights from China to U.S. military bases, where they have been subject to extensive screening and 14-day quarantines.
Cruise ships have also been quarantined and isolated, leaving thousands of passengers stranded.
The Associated Press contributed to this report.