U.S. Health Secretary Alex Azar warned Tuesday that the coronavirus raging across mainland China is a "potentially very serious public health threat," saying that the Trump administration is expanding screening for the virus from five to 20 U.S. airports. "We are constantly preparing for the possibility that the situation could worsen," Azar said during a press briefing on the nation's response to the coronavirus, which has killed 106 people in mainland China and infected nearly 4,700 worldwide, including in the United States. "Americans should not worry about their own safety," Azar said. "Part of the risk we face is we don't know everything we need to know about this virus ... That does not prevent us from preparing and responding." The U.S. Centers for Disease Control and Prevention earlier in the day advised Americans to avoid all non-essential travel to China, expanding its travel warning from the city of Wuhan, the epicenter of the outbreak, to the entire country.

Passengers at LAX after arriving from Shanghai, China, after a positive case of the coronavirus was announced in the Orange County suburb of Los Angeles, January 26, 2020. Ringo Chiu | Reuters

"To better protect the health of the American public during the emergence of this novel coronavirus and based on the evolving information from China, CDC has reassessed its entry strategy," CDC Director Robert Redfield said at the press briefing. CDC officials said the screenings will focus on identifying ill patients as well as educating travelers so they can help contain the disease. [WATCH: the HHS press briefing here.] Coronaviruses are a large family of viruses that usually infect animals but can sometimes evolve and spread to humans. Symptoms in humans include fever, coughing and shortness of breath, which can progress to pneumonia. U.S. health officials said Tuesday that they are still determining the real speed and spread of the disease and whether it can infect people prior to symptoms. Azar said the so-called R naught, a mathematical equation that shows how many people will get an illness from each infected person, appears to be somewhere around 1.5 to 3. Azar said symptoms from the new virus, temporarily named 2019-nCoV, may take up to 14 days to appear. Azar said he offered to send CDC officials to China to assist public health officials there. "We are urging China [to act] with more cooperation and transparency," he said.