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.”

The risk of Ebola infection remains vanishingly small in this country. The virus is not airborne, not able to travel in the way that, say, measles or the SARS virus can. Close contact with a patient is required for transmission. Just one death from Ebola has occurred here, and medical care is light-years from that available in West Africa, where more than 4,400 people have died in the latest outbreak.

By contrast, in some years, the flu kills more than 30,000 people in the United States. Yet this causes little anxiety: Millions of people who could benefit from a flu shot do not get one.

“We’re familiar with the flu, we’ve had it and gotten better — we feel we know that threat,” Dr. Slovic said.

Experts said the most recent precedent of the Ebola risk, psychologically speaking, is the anthrax scare that followed the Sept. 11 attacks. In the weeks after an unknown assailant sent deadly envelopes with powdered anthrax spores to public officials, people across the country were seized by anxiety.

Some duct-taped windows and stayed away from work. In pockets of the country — Tennessee, Maryland and Washington — people reported physical symptoms like headaches, nausea and faintness. Ultimately they were determined to be the result of hysteria.

“I was in college then, and I remember they evacuated the business school building because someone saw white powder in the cafeteria,” said Andrew Noymer, a sociologist at the University of California, Irvine. The powder turned out to be artificial sweetener.

Ebola has arrived at a different cultural moment. In 2001, fears of terrorism were all too real. Still, perception of risk is far from a strictly rational calculation.

Psychologists have known for years that people judge risk based on a sophisticated balance of emotion and deduction. Often the former trumps the latter.

Instinctual reactions are quick and automatic, useful in times when the facts are not known or there is not enough time to process what little is known. Analytical reasoning is much slower and much harder; if we relied on analysis alone, decisions about risk would paralyze us.