America has its killer bugs, but Americans don't, as a rule, express great concern about them: Pneumonia, which killed 63,000 Americans in 2000, draws little public comment. Until 2003, when the flu deaths of 20 U.S. children early in the season were widely publicized, Americans didn't worry much about influenza either, despite the tens of thousands of deaths attributed to that disease each year.

In comparison, relatively minor threats are widely feared. First publicized in 2002, West Nile virus was perceived as a great threat, though it killed only 284 people in the United States. In 2003, when severe acute respiratory syndrome, or SARS, emerged in Asia, there were only 7,000 cases in the world and fewer than 100 in the United States. No one in the United States died of SARS, but a lot of people worried. Many patients called my internal medicine practice in New York convinced that the slightest cough was SARS. People were afraid to sit next to an Asian person or to eat in a Chinese restaurant.

Each terror alert, too, triggers a wave of often unjustified fear. Anthrax infected 22 people through the U.S. mail in the fall of 2001, killing five. Yet 30,000 people began taking the powerful antibiotic Cipro, many indiscriminately and without a doctor's prescription. In 2002 news reports began to circulate that some of the smallpox virus that had been stockpiled in the former Soviet Union might have found its way into the hands of rogue dictators or terrorists. There hasn't been a case of smallpox in the United States since the 1940s and if it ever reappears it is likely to spread slowly, but the fear of smallpox raced virulently through the public.

Of course we can't just turn off fear; it's part of our psychological makeup from birth, integral to the elaborate system of self-protection that has preserved the human species for millenniums. But lately, you could argue, the fear component of that system has been malfunctioning.

Over the past century, we Americans have dramatically reduced our risk in virtually every area of life, resulting in life spans 60 percent longer in 2000 than in 1900.

It used to be that a person could die from a scratch. Now we take effective antibiotics at the first sign of trouble. Public health measures dictate standards for drinkable water and breathable air. Our garbage is removed quickly. We live temperature-controlled, largely disease-controlled lives.

And yet, we worry more than ever before. The natural dangers are no longer there, but the response mechanisms are turned on much of the time. We implode, turning our adaptive fear mechanism into a maladaptive panicked response.

The irony is, by worrying so much about our health we jeopardize our well-being, increasing our risk for conditions including heart disease, cancer, stroke and depression.

What Has Gone Awry?

The mass media tend to magnify the latest health concern and broadcast it to millions of people at once. This has the effect of provoking panic way out of proportion to the risks. I call this phenomenon the "bug du jour." The craze of the moment appears to be a threat until it runs its course through the media spotlight.

While our sense of threat has grown, some of our safe havens have disappeared. The health authorities we used to look to in the community have been replaced by conglomerates that thrive in a climate of uncertainty. The leather-handed neighborhood pharmacist who suggested what type of condoms or deodorant to buy has been replaced by anonymous clerks in a 24-hour, neon-lit chain store.

Fear -- a physical reaction to a perceived threat -- is programmed into our being. When an individual feels threatened, fear revs up the metabolism in anticipation of an imminent need to defend oneself or flee.

The acute stress response, popularly known as "fight or flight," was first described by American physiologist Walter Cannon in the 1920s. Cannon observed that animals, including humans, react to threats with a hormonal discharge of the nervous system. There is an outpouring of vessel-constricting, heart-thumping hormones, including noradrenaline (norepinephrine) and adrenaline (epinephrine), followed by the steroid cortisol. The heart speeds up and pumps harder, the nerves fire more quickly, the skin cools and gets goose bumps, the eyes dilate to see better and the brain receives a message that it is time to act.

Accepting ourselves as animals is crucial to understanding how fear is designed to work. The fear center of the human brain, the amygdala, is identical to the fear center of the animal brain.

New York neuroscientist Joseph Ledoux, whose book "The Emotional Brain" describes how the brain processes emotions, has studied fear extensively. "When a rat is afraid and when a human is afraid, very similar things occur in the body," he told me. But Ledoux also believes that triggers of fear vary dramatically from species to species.

Ledoux described the amygdala, the almond-shaped brain structure that interprets emotion, as "the hub in the brain's wheel of fear." When the amygdala is stimulated, there is an outpouring of stress hormones, causing a state of hyper-vigilance. The amygdala processes the primitive emotions of fear, hate, love, bravery and anger -- all neighbors in the deep limbic brain that we derive from lower animals. When the amygdala malfunctions, a mood disorder, or state of uncontrollable apprehension, results.

The amygdala works together only with other brain centers that feed it and respond to it. This fear hub senses via the thalamus (the brain's receiver), thinks through the cortex (the brain's seat of reasoning) and remembers via the hippocampus (the brain's file cabinet).

This cycle can become self-perpetuating. My young son, for example, has been afraid of dogs since he was terrified at the age of 2 by a sudden barking he heard when we were hiking on a mountain trail. I said, "The dog is gone," but he said, "No, it's not. It keeps coming back."

Now each time a dog barks, it engages the same mechanism. My son's thalamus triggers his amygdala, which retrieves the fearful memory for him from the hippocampus, and his body goes into hyper-drive. This is a warning system malfunction, because it is alerting him to a danger that doesn't actually threaten him.

Once a person has learned to fear something, he may always feel fear associated with that experience. But unlike mice, we humans can fear events we have only read or heard about, and so we worry about disasters we may never experience. If we are unable to respond for lack of an appropriate target, the fear accumulates, and we become anxious.

The Wrong Worries

Many of the bugs du jour are cause for concern only among a narrow segment of the population. Only a small portion of those who believe they are at risk really are, and few who become infected actually die. But a strange disease that kills only a few people still makes for good headlines.

This year the concern is bird flu, and more than 2 million doses of a hastily prepared vaccine will be discarded if the dread virus doesn't mutate to a form that can infect us. Since this mutation occurs once in 50 years, the current worry may well turn out to be overblown.

At a time in history when true scourges are quite rare, the population is controlled by fear. Rather than enjoy the safety that our technological advances have provided us, we feel uncertain. Respiratory masks and other paraphernalia meant to shield us actually spread panic more effectively than any terrorist agent by sending the message that something is in the offing. Our personal fear alerts are turned on all the time.

We feel the stress and become more prone to irritability, disagreement, worry, insomnia, anxiety and depression. We are more likely to experience chest pain, shortness of breath, dizziness and headache. We become more prone to heart disease, cancer and stroke, our greatest killers.

Worry about the wrong things puts us at greater risk of the diseases that should be concerning us in the first place.


Marc Siegel, an associate professor of medicine at the New York University Medical School, last wrote for Health about the search for a better ovarian cancer screen. This article is adapted with permission of the publisher John Wiley & Sons, Inc. from Siegel's "False Alarm: The Truth About the Epidemic of Fear," copyright 2005.