Dr. Messonnier called the federal order on Friday “science-based,” saying it was warranted because of the worsening epidemic in China. She cited the widening, rapid spread of the disease there, the increase in person-to-person transmission and the rising death toll. In addition, evidence has been emerging that people can transmit the disease even before they show symptoms of the illness, which can cause pneumonia in severe cases.

Another concern was that the passengers evacuated had been in Wuhan, the epicenter of the outbreak. At a news conference later on Friday, Alex M. Azar II, the secretary of health and human services, announced that other citizens of the United States who were returning from Hubei Province, which includes Wuhan, would also be quarantined for up to two weeks. Mr. Azar declared a public health emergency for the United States, and said foreign nationals who had been to China during the last 14 days would not be allowed into the United States.

“This is a very serious public health situation, and the C.D.C. and federal government have and will continue to take aggressive action to protect the public,” Dr. Messonnier said. “If we take strong measures now, we may be able to blunt” the impact of the virus in the United States.

Dr. Messonnier said that the people who were quarantined did not pose a threat to the surrounding community.

Since the 195 evacuees arrived in Riverside, they have had their noses and throats swabbed to test for the virus, and their temperatures taken several times a day. They were initially told that they would be detained for at least 72 hours, and possibly 14 days. Two weeks is thought to be the upper limit of the incubation period; if symptoms don’t start by then, a person can be cleared as not infected.