A fourth U.S. case of the deadly new coronavirus that has killed more than 50 people in China has been confirmed in Southern California, health officials said Sunday.

The Los Angeles County Department of Public Health has confirmed the first case of the virus in the county. The infected person presented themselves for care once they noticed that they were not feeling well and was receiving medical treatment, the department said in a statement.

"There is no immediate threat to the general public, no special precautions are required, and people should not be excluded from activities based on their race, country of origin, or recent travel if they do not have symptoms of respiratory illness," the department said.

The department said that residents, students, workers, and visitors should continue to engage in their regular activities – and practice good public health hygiene "as this is the height of flu season."

The department said it was working closely with the Centers for Disease Control and Prevention and other federal, state and local agencies to monitor the "rapidly evolving situation.

The announcement came one day after the neighboring Orange County Health Care Agency’s Communicable Disease Control Division said an individual was diagnosed with the virus there. That patient, hospitalized in isolation in good condition, had traveled from Wuhan, a Chinese city of 11 million people that is the outbreak's epicenter.

"The current risk of local transmission remains low," the agency said in a statement. "There is no evidence that person-to-person transmission has occurred in Orange County."

The CDC says people who have casual contact with a case, such as in the same grocery store or movie theater, are at minimal risk of developing infection.

The first U.S. case, a Washington state man in his 30s, was diagnosed Tuesday. A second case, a woman in her 60s, was reported Friday in Chicago.

The CDC says more U.S. patients are likely. The virus, first diagnosed in China last month, has infected at least 1,975 people and killed 56 people there, according to that nation's National Health Commission.

China Health Minister Ma Xiaowei said Sunday that it appeared the "ability of the virus to spread is getting stronger” and that it is infectious before symptoms show, a characteristic that could make it harder to contain.

“From observations, the virus is capable of transmission even during incubation period” of one to 14 days, Ma said. “There are hidden carriers.”

That makes this outbreak different from severe acute respiratory syndrome, SARS, a virus that was not contagious during incubation. SARS killed more than 600 people across mainland China and Hong Kong along with more than 100 other people around the world in 2002-2003.

Ogbonnaya Omenka, an assistant professor and public health specialist at Butler University's College of Pharmacy and Health Sciences, says immediate reporting of symptoms or contact is now more crucial than ever.

"Asymptomatic transmission of the novel coronavirus definitely makes it more problematic and more difficult to contain," Omenka told USA TODAY. "This automatically heightens the level of concern about the existing cases in the United States. Could there be other cases?"

The World Health Organization on Thursday declined to categorize the coronavirus as a global health emergency, lauding China for its efforts to contain the virus.

Chinese authorities have locked down Wuhan and at least two cities in an unprecedented effort to contain the virus during the busy Lunar New Year travel period. The U.S. closed its consulate in Wuhan and was pulling out its diplomats.

A small number of cases have also been diagnosed in Hong Kong, Macao, Thailand, Taiwan Japan, South Korea, Vietnam, Singapore, Malaysia, Nepal, France, Canada and Australia.

Coronavirus death toll rises to 56 in China: US diplomats prepare to leave Wuhan

Coronaviruses are a large family of viruses that cause illness ranging from the common cold to pneumonia. Common signs of infection include fever, cough, shortness of breath and breathing difficulties. In more severe cases, infection can cause severe acute respiratory syndrome, kidney failure and death.

Coronaviruses are zoonotic, meaning they are transmitted between animals and people. Many of the initial cases were linked to a seafood and meat market in Wuhan. Chinese health officials said human-to-human transmission has been confirmed.

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