(FORTUNE Magazine) – MY OFFICE FINALLY GOT ITS SUPPLY OF FLU VACCINE THIS week--later than usual but in plenty of time for flu season, which usually doesn't start until late December. Yet being offered a regular flu shot doesn't seem to reassure my patients. They're worried about bird flu. Last Thursday was typical: Five patients begged for prescriptions for the antiviral drug Tamiflu, and two asked for a shot of bird flu vaccine, which doesn't yet exist. The last patient of the day muttered as he was leaving, "Bird flu is going to get us all this year."

The greatest problem my patients have isn't influenza--it is fear of influenza. Antivirals like Tamiflu are props against that fear, placebos as much as cures. Bird flu is indeed a threat--especially if you have wings. It has shown itself to be a ruthless killer that chokes off the breathing of millions of birds, and it has prompted humans to slaughter millions more. But for people, the risk so far is close to zero for anyone except bird handlers in affected regions. Up until now, the virus has killed fewer than 65 people worldwide, and it will not spread widely among humans unless there are several more mutations of its DNA.

Most of my patients understand all this, and they know just as well as I do that there is no bird flu in the U.S. What scares them is the possibility of a pandemic. It's remote, but it's real--the hardest kind of risk to put in perspective. Yet focusing on a worst-case scenario can have grave consequences. In public policy, it means that more than $2.2 billion of the $7.1 billion in anti-pandemic funding requested by President Bush is earmarked for stockpiling vaccines and Tamiflu--perishable goods that would be of use only if a pandemic occurs in the next three years. That's nearly as much money as the plan would devote to potentially more useful research geared toward modernizing vaccine making (which in the age of genetic engineering still relies on the slow, old-fashioned practice of growing vaccines in chicken eggs). Meanwhile, worst-case news reports, including constant comparisons to the terrifying flu pandemic of 1918, have scared my patients out of all proportion. The odds remain overwhelmingly on their side: In 1918 there was no modern science. There were no vaccines for flu or antiviral drugs, no antibiotics or steroids to treat flu patients with pneumonia or asthma, no world-spanning public-health network. But instead of this reassuring information, our TV screens continually project images of beleaguered Asians and stricken birds. We attach ourselves voyeuristically to the news, often reported by sincere people who are overly afraid themselves.

Our own brains make us easy prey to such distortions. Dr. Elizabeth Phelps, neuroscientist at New York University, has examined how the brain responds to envisioned threats. Using highly sensitive MRI scans, she discovered that the brain's fear center (known as the amygdala) can be activated in response to dangers a person merely observes. "When you are watching it and you are told that it is going to happen to you, it causes the same robust response by the amygdala as if you actually experience it yourself," Phelps says. She has also studied the safety signals that our brains use to turn off fear; it turns out the brain's wiring heavily favors the "on" switch.

That leaves modern humans forever in the position of trying to turn off a response that often shouldn't have been turned on in the first place. Fear is meant to warn us against looming peril, not to provoke a generalized response to sensational videos of foreign lands. But since our fear radar doesn't discriminate, we release stress hormones unnecessarily, readying for a crisis that doesn't come. Heart rates and blood pressure increase, we breathe harder, and like a car that revs constantly at high speed, we are more likely to break down. What bothers me most as a physician is that I see my patients being harmed, and there's little I can do to stop it. That is a chilling foretaste of the horror of a true epidemic.

■ MARC K. SIEGEL, MD, is author of False Alarm: The Truth About the Epidemic of Fear (Wiley, 2005). FEEDBACK fortunemail_letters@fortunemail.com