Two coronavirus patients from San Benito County were transferred to UC San Francisco Monday for treatment.

The husband and wife, both 57, were initially placed in isolation in their home, but began showing worsening conditions.

“It was then determined that both patients needed to be admitted to a hospital equipped for a higher level of care. The patients have been transferred out of San Benito County by specialty ambulance,” Dr. Marty Fenstersheib, San Benito County health officer, said in statement.

The patients are being cared for in isolation by UCSF Health, which specializes in treating infectious diseases. It also admitted patients during past epidemics, such as SARS in 2003. There are multiple branches of UCSF in San Francisco and it's unknown which location is treating the couple.

"The risk to San Francisco residents remains extremely low," said Dr. Charles Chiu, a professor of laboratory medicine and infectious disease at UC San Francisco. "At this time, there is no evidence of sustained transmission of the coronavirus in the United States. All of the 11 confirmed cases to date in the United States (6 in California) have either been returning travelers from Wuhan, China or close household contacts."

The man, who traveled to Wuhan, tested positive for the virus and subsequently transmitted it to his wife, according to officials. The names of patients aren't being released for privacy reasons.

They are the first California case involving person-to-person transmission of the pneumonia-like virus

ALSO: Map: Where 11 cases of coronavirus are confirmed in the US

There are now 11 cases of the new coronavirus confirmed in the United States. Six are in California, two are in Illinois and there is one each in Arizona, Massachusetts and Washington state, according to the Centers for Disease Control and Prevention. Nine of the 11 people with the virus had recently traveled to Wuhan. There are two cases of transmission between spouses.

The virus has infected more than 20,000 people and killed more than 400 people in China after originating in the city of Wuhan in December. Travelers from China — either residents or foreign visitors to Wuhan— have introduced the pneumonia-like virus to two dozen countries.

The World Health Organization said Monday that outside of China, there are 153 confirmed cases, including one death in the Philippines and another in Hong Kong.

Amy Graff is a digital editor with SFGATE. Email her: agraff@sfgate.com.