As health officials scramble to explain how two nurses in Dallas became infected with Ebola, psychologists are increasingly concerned about another kind of contagion, whose symptoms range from heightened anxiety to avoidance of public places to full-blown hysteria.

So far, emergency rooms have not been overwhelmed with people afraid that they have caught the Ebola virus, and no one is hiding in the basement and hoarding food. But there is little doubt that the events of the past week have left the public increasingly worried, particularly the admission by Dr. Thomas R. Frieden, director of the Centers for Disease Control and Prevention, that the initial response to the first Ebola case diagnosed in the United States was inadequate.

On Wednesday, the C.D.C. offered up the latest piece of bad news, announcing that a second infected nurse in Dallas had flown back from Cleveland a day before developing symptoms. Even before the announcement, two-thirds of the respondents to a Washington Post-ABC News poll said they were concerned about a widespread epidemic of Ebola in this country.

Experts who study public psychology say the next few weeks will be crucial to containing mounting anxiety. “Officials will have to be very, very careful,” said Paul Slovic, president of Decision Research, a nonprofit that studies public health and perceptions of threat. “Once trust starts to erode, the next time they tell you not to worry — you worry.”