“This is the third case identified in our county. But it's different from the other two cases in an important way ...Our third case did not recently travel overseas or have any known contact with a recent traveler or infected person,” Dr. Cody told an afternoon news conference.

The latest confirmed case of coronavirus in the United States, diagnosed in a California woman, is of “unknown origin” and was found in a patient who had not traveled overseas or had contact with any known travelers, local health officials said on Friday.

The woman, identified only as an adult with chronic health conditions, was referred by her physician for testing after she developed difficulty breathing, said Dr. Sara Cody, chief health officer for Santa Clara County.

Santa Clara is home to the Silicon Valley tech hub in Northern California.

“This is the third case identified in our county. But it's different from the other two cases in an important way ...Our third case did not recently travel overseas or have any known contact with a recent traveler or infected person,” Dr. Cody told an afternoon news conference.

It is also the second case of a patient who contracted the virus from an unknown origin. On Wednesday, health authorities announced a woman in Solano County, California, about 145 km to the north, was the nation's first case of community transmission.

It means the virus is now spreading within at least two separate locations in California.

Dr. Cody said that the discovery of a second patient suffering from coronavirus from unknown origins marked a new chapter for public health officials where quarantines and travel restrictions were no longer enough to contain the virus.

“We will continue to trace close contacts of our cases to try to limit the spread of the virus. But now we need to add other public health tools to the mix,” Dr. Cody said.

“Now that we have a case that did not recently travel or come in contact with anyone known to be ill, what does this mean? What we know now is that the virus is here, present at some level. But we still don't know to what degree,” she said.

The number of confirmed U.S. cases of the respiratory disease is still relatively small at roughly 60, most of them repatriated American passengers from the Diamond Princess cruise ship docked in Japan, but there are growing fears that the country is on the cusp of wider outbreak.

The U.S. State Department has issued travel advisories for several nations and on Friday it said Americans should reconsider travel to Italy, where nearly 900 coronavirus cases have been confirmed.

A similar warning was issued earlier this week for South Korea, which with more than 2,300 cases, has the largest outbreak outside China.

The United States will make a decision “very soon” about whether to bar entry to travelers from the countries with the most cases of the new coronavirus, U.S. President Donald Trump said.