The United States has seen the first reported case of Ebola outside Africa, brought by a Liberian national who traveled to Dallas two weeks ago. Airports in West Africa have promised to screen every passenger attempting to get on a plane, which has left many Americans wondering: Just how did an Ebola patient make it through four airports without being stopped?

Ebola symptoms can lay dormant for two to 21 days, and during that time, the disease wouldn't show up on a blood test, let alone a thermometer. There was no way to know that this particular passenger was at risk. The good news, says Thomas Frieden, director of the CDC: "Ebola doesn't spread before someone gets sick, and he didn't get sick till four days after he got off the airplane. So we do not believe there is any risk to anyone who was on the flight at that time."

Fever is one of the first symptoms to appear, so for now, thermometers remain a good way of catching infected travelers.

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