Since officials first reported the coronavirus outbreak in Wuhan in late December and the United States imposed a travel ban for those entering from China, Border Patrol agents have detained 333 Chinese nationals attempting to enter the United States illegally, according to Department of Homeland Security data obtained by Fox News.

While none have tested positive for the virus, the southern border remains a primary focus of the Trump administration, which sees the area as high risk and a gateway for COVID-19 to spread in the U.S.

"We have a unique public health threat posed by individuals arriving unlawfully at the border, where migrants, law enforcement officials, frontline personnel, and the American public are put at risk," said a DHS official who asked to remain anonymous because he was not authorized to speak publicly by the agency.

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Under MPP, which stands for the Migrant Protection Protocol, the U.S. has deported some 60,000 migrants to Mexico to await their deportation hearings. Unless the Supreme Court acts, the policy will be ruled illegal in California and Arizona and those individuals and other migrants claiming asylum will be allowed to remain in the U.S. indefinitely.

"All it would take is a single infected individual to impact the detained migrant community within DHS facilities without proper precautions, which can only happen through orderly, lawful migration, the virus threatens to spread rapidly,” the official said. “Any halting of MPP would exacerbate this threat."

The U.S. southern border is a microcosm of the world, with foreign nationals from 122 separate countries apprehended or denied entry already this fiscal year -- from October through the end of February.

Seventy of those countries currently report confirmed COVID-19 cases, led by China but followed by Italy, Iran, India, Romania, Vietnam and Brazil.

The virus started popping up on health officials’ radar screens in late December in China and has since spread around the world. Some of the countries on the list have had very low rates of the virus, including Mexico.

Meanwhile, the number of cases in the U.S. has risen dramatically in recent days, topping 1,000. China has nearly 81,000 confirmed cases of coronavirus, Italy has more than 10,000 and Iran has about 8,000.

Last week the Border Patrol asked the CDC to take over testing of those migrants who show symptoms consistent with COVID-19. U.S. Immigration and Customs Enforcement also runs 16 detention facilities with airborne infection isolation rooms. So far, it tested 4 migrants, all coming back negative.

"It's absolutely a risk that we're monitoring and we're evaluating," U.S. Customs and Border Protection Acting Commissioner Mark Morgan told Fox News. "And if it shifts and we're asked to do more at the SW border, we're ready to operationalize that.”

Agents say the threat is threefold.

One, Border Patrol agents can't work from home or practice social distancing. Those who work in the field are exposed to human contact every day with dozens they have never met, often in poor physical shape, coughing and sneezing.

Secondly, unlike legal immigration, where tourists and migrants arrive with a passport and travel history, migrants arriving illegally have no medical history and no travel manifest, allowing agents to determine what countries they visited and how long.

Frequently, those apprehended stayed in stash houses in Mexico with many others in the days ahead of crossing. None of those people know where the others came from or who they traveled with, whether sick people preceded them in the houses, who may have slept in that spot the night before, whether it was cleaned.

Finally, Border Patrol detention facilities are crowded, usually overflowing with migrants stuffed into pods for several days awaiting immigration hearings or transport to an airfield, community center or deportation to Mexico.

These detention centers are breeding ground for many types of communicable diseases: chickenpox, measles and the flu. In May of last year, a detention center in Texas had to be isolated and shut down after nearly three dozen detainees became ill with the flu and a 16-year-old boy died.

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In June, ICE reported 297 cases of the mumps in 39 detention facilities as the disease spread across several states.

As for COVID-19, last week DHS acting Secretary Chad F. Wolf told the House Homeland Committee: "The administration will continue to closely monitor the virus globally, as well as in our hemisphere, and will adjust our proactive measures as necessary."

Roughly, 1,000 migrants a day are caught attempting to enter illegally from Mexico, which has detected seven coronavirus cases. President Trump considered closing the border, but has since downplayed the idea, while still using the virus as a talking point to support his call for a wall.