Health officials are preparing for the possibility of a pandemic as coronavirus cases in China top 20,000. There are now 425 deaths related to the coronavirus since the outbreak began.

The United States is planning more flights to evacuate American citizens out of Wuhan, China, this week, according to the U.S. embassy in Beijing.

Those flights will land at four U.S. military bases, and similar to the evacuation flight that landed in California last week, passengers will be quarantined upon arrival. The planes will be loaded with medical supplies and humanitarian goods, which the U.S. hopes to deliver to Wuhan on the first leg of the journey.

At Princeton University, more than 100 students self-quarantined on Sunday because they’d recently traveled to China, a university spokesperson confirmed. That figure is now down to fewer than 20, according to a statement from the university Monday night. The school directed students, faculty and staff who traveled to mainland China to self-isolate for 14 days after their return. Depending on their housing situation, students in self-quarantine have had dining services bring them food and laundry arranged by the university. The university is also working to video conference quarantined students into class. The number of students in self-quarantine is expected to fall in the coming days, the spokesperson said.

In Wuhan, the epicenter of the outbreak, Justin Steece, a 26-year-old American, made a video diary documenting his experience. Steece, who works in Wuhan and whose wife is a Chinese national, said his family was denied an evacuation flight by the State Department because his infant son doesn’t yet have a passport. Chinese state-run media said there are 15 more coronavirus cases in Hong Kong and that 632 people admitted to the hospitals with the virus have been discharged.

Meanwhile, top U.S. health officials pushed back on the notion that its new travel ban against Chinese nationals and withdrawal of embassy staff was spreading fear about coronavirus.

“This is an aggressive action by the United States, but our goal is to slow this thing down,” Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases, said at a Monday news conference.

Messonnier’s response came hours after Chinese foreign ministry spokesperson Hua Chunying called the United States’ actions “excessive” and said that such measures “could only create and spread fear.”

“This is an unprecedented situation,” Messonnier said of the novel coronavirus outbreak.

She cited the outbreak’s rapid expansion, lack of population immunity to the new virus, person-to-person and community transmission in China, and concerning data about possible asymptomatic disease transmission as factors that went into health officials’ decision-making process.

“All of those are worrisome data points,” Messonnier added.

U.S. coronavirus patients’ illnesses range from mild to severe

The U.S. currently has 11 patients who have tested positive for the new coronavirus and 167 patients who have tested negative. Eighty-two tests are still pending, according to the Centers for Disease Control and Prevention.

Of those cases, nine are travel-related and two involved human-to-human transmission between close contacts — in each instance, the transmission was between a husband and wife.

According to Messonnier, the U.S. coronavirus cases have been along a spectrum of severity. Some of the cases have been mild. Other patients have had moments when they were “extremely ill” and required oxygen to breathe. Data out of China suggests that people who are older, or who have underlying health problems, are at higher risk for severe forms of coronavirus.

No deaths have been reported in the United States.

The coronavirus outbreak has now infected more than 20,000 people globally and has killed at least 425 of them, almost all in China.

Beyond China, at least 169 cases have been confirmed in 25 countries.

The vast majority of cases of the novel coronavirus have been reported in mainland China, with the epicenter still in Wuhan, a city of 11 million and the capital of central Hubei province, where the first cases were detected back in December. A total of 17,205 people have been infected with the disease and 361 have since died, according to the National Health Commission of the People’s Republic of China.

In the first case of death outside China, a man in the Philippines died after contracting the coronavirus from his friend while they were traveling together in Wuhan, China, according to the Philippines Department of Health.

Hong Kong expands border closures

Hong Kong leader Carrie Lam on Monday announced additional border closures, severing all but three links between the semi-autonomous Chinese city and mainland China. The Hong Kong International Airp[ort, the Shenzhen Bay border and the Hong Kong-Zhuhai-Macau bridge remain open.

The move came as thousands of public hospital workers went on strike Monday morning, demanding the Hong Kong government shutter all borders with mainland China as the country struggles to contain the outbreak. At least 15 people in the city have been infected with the novel coronavirus, according to the Hong Kong Department of Health.

U.S. declares emergency and travel ban

The U.S. Secretary of Health and Human Services Alex Azar has declared a public health emergency and a temporary travel ban.

Over the weekend, the U.S. Department of Homeland Security announced that flights entering the country from China would be rerouted — at no additional cost to the passenger — to seven airports designated for screenings.

The new coronavirus causes symptoms similar to pneumonia that can range from mild, such as a slight cough, to more severe, including fever and difficulty breathing, according to the U.S. Centers for Disease Control and Prevention.

ABC News’ Joseph Simonetti contributed to this report.