At the nation’s borders, federal workers keep the country safe in many ways: Some investigate sick passengers. Some examine animals for dangerous pathogens. And some inspect plants for infestations that could spread in this country.

Late last week, the Trump administration dispatched hundreds of those federal employees with the same message that colleagues at other agencies received: Their services were no longer needed.

The absence of these federal officers at the borders leaves Americans vulnerable to pathogens carried by plants, animals and people, experts warned.

The firings come even as the Trump administration is said to be readying plans to turn back migrants on the grounds that they might bring diseases like tuberculosis and measles into the country.

“Screening for communicable diseases at ports of entry is an important role of public health in order to prevent communicable diseases from entering our country,” said Dr. Carlos del Rio, an infectious disease physician at Emory University.

“Not having public health employees to do this job is concerning and makes us less safe,” he added.

The Centers for Disease Control and Prevention estimates that every day, nearly 30,000 planes travel in and out of the country. In 2019, more than 400 million travelers arrived via more than 300 ports of entry. About half of those people crossed the border between the United States and Mexico.

Border posts are manned by workers from multiple agencies. Employees from the C.D.C.’s Division of Global Migration Health screen people, animals and animal products for diseases, respond to reports of ill travelers and distribute medications as needed.

The Trump administration last week dismissed an unknown number of people from the C.D.C.’s 20 port health stations, leaving some entirely unattended, according to three officials with knowledge of the situation.

Calls to the port station in San Juan, P.R., on Wednesday, for example, were rerouted to the station in Miami, where a C.D.C. employee who declined to be identified said that no one would be at the San Juan post “for a very long time.”

Other port stations are in Anchorage, Atlanta, Chicago, New York, San Francisco and three cities in Texas.

C.D.C. officers can legally detain or conditionally release people and wildlife suspected of carrying any disease on a long list that includes measles, tuberculosis, pandemic influenza and viral hemorrhagic fevers such as Ebola and Marburg.

The Department of Agriculture employs entomologists, botanists and mycologists — experts in insects, plants and fungi, respectively — who inspect agricultural products for pests and pathogens. Many of those specialists were also let go on Friday.

“We’re such a critical program, it makes no sense,” said one U.S.D.A. official with knowledge of the situation, who asked not to be named for fear of retaliation.

“If we don’t work and those inspections don’t happen, things start piling up at the ports,” the official added.

Animal health experts have been particularly concerned about African swine fever and New World screwworm, two diseases that have been creeping closer to the United States and could have devastating impacts on the pork and beef industries.

On Tuesday, the U.S.D.A. said it had mistakenly fired several employees working on the country’s bird flu outbreak and was trying to hire them back, according to a report by NBC News. It’s unclear how many of the employees have returned to their positions.