But these temperature checks aren’t always effective. In the Dallas case, the man left Liberia on September 19, had his temperature checked at the airport, and arrived in America on September 20. He only developed symptoms on the 24th, however, and he was isolated four days later. Patients are only contagious when they’re symptomatic, so there’s no risk the people on the flight with the man caught Ebola. There is, however, a four-day window in which he might have infected others in the U.S.

In July, an Ebola-infected man flew from Liberia to Nigeria, even though airport screenings were already in place, and he infected healthcare workers in Lagos. And of course, fevers are usually caused by flu and other illnesses that aren’t Ebola.

Twenty different ports of entry in the U.S. have been equipped with quarantine centers that are on the lookout for passengers with Ebola-like symptoms. “If you’re a passenger on a plane and you say you’re sick, you will be met when you land by the CDC,” agency spokesman David Daigle told The New York Post.

At arrival gates, border protection officers keep their eyes peeled for passengers who show signs of fever, sweating, or vomiting.