IN THE MIDDLE of a disease outbreak, panic and fear can themselves be a destructive force. When the Ebola virus first broke out in West Africa, governments dropped the ball and were unable to contain the dread, leading people to take action — such as evading quarantine — that spread the infections. A core requirement of managing a crisis like this is that public health officials and political leaders maintain the public’s confidence.

This premise applies just as well to the United States as it does to Africa, and in recent days, a significant misstep in Dallas has shaken that confidence.

A man from Liberia, Thomas Duncan, had contact with a pregnant woman there who was infected, although he may not have known that when he carried her to a hospital. She later died. He did not show symptoms immediately, and went to the airport, where he signed a screening form saying he had not had contact with anyone with Ebola, and flew to the United States. Once in Dallas, Mr. Duncan began to feel ill and went to the emergency room at Texas Health Presbyterian Hospital. While there, he came in contact with other people who were healthy. He was released by the hospital, which concluded he had only a low-grade fever from a viral infection. The hospital apparently did not connect the dots of his illness and his recent travel from Liberia, one of the nations most seriously hit by the recent outbreak. Mr. Duncan then returned to the hospital when his symptoms worsened and officials confirmed he was infected with the Ebola virus.

The misstep was the failure in the Dallas emergency room to realize that he was a potential Ebola victim, allowing him, at least theoretically, to come in contact with others present and after he was released. The virus does not transmit through the air, only through bodily fluids, and is not contagious if the person is not symptomatic, as was the case while Mr. Duncan was flying to the United States. But on his first visit to the emergency room, he was showing symptoms and should have been immediately isolated and tested. The key to fighting Ebola — for which there are no readily-available vaccines or therapeutics — is identifying those infected, isolating them and tracing those they have been in contact with. This kind of vigilance was not on display in the hospital emergency room.

Screening airline passengers is certainly useful, but it is not foolproof, since Ebola doesn’t always show symptoms right away. This is unlike influenza, which is more readily detectable. A screening form did not stop Mr. Duncan from boarding a plane to the United States.

Ebola presents a frightening challenge from nature, but it can be stopped. The United States enjoys one of the most advanced and robust health-care systems in the world, capable of isolating Ebola patients when they arrive and halting transmission. But it is going to require zero tolerance for oversights like that which appears to have occurred in Dallas. Other U.S. hospitals — including those in the Washington area evaluating possible Ebola cases — must take a lesson.