EDITOR'S NOTE: Dr. Welch is the author of "Should I Be Tested for Cancer? Maybe Not and Here's Why" (University of California Press). Dr. Schwartz and Dr. Woloshin are senior research associates at the VA Outcomes Group in White River Junction, Vt.

For most Americans, the biggest health threat is not avian flu, West Nile or mad cow disease. It's our health-care system.

You might think this is because doctors make mistakes (we do make mistakes). But you can't be a victim of medical error if you are not in the system. The larger threat posed by American medicine is that more and more of us are being drawn into the system not because of an epidemic of disease, but because of an epidemic of diagnoses.

Americans live longer than ever, yet more of us are told we are sick.

How can this be? One reason is that we devote more resources to medical care than any other country. Some of this investment is productive, curing disease and alleviating suffering. But it also leads to more diagnoses, a trend that has become an epidemic.

This epidemic is a threat to your health. It has two distinct sources. One is the medicalization of everyday life. Most of us experience physical or emotional sensations we don't like, and in the past, this was considered a part of life. Increasingly, however, such sensations are considered symptoms of disease. Everyday experiences like insomnia, sadness, twitchy legs and impaired sex drive now become diagnoses: sleep disorder, depression, restless leg syndrome and sexual dysfunction.

Perhaps most worrisome is the medicalization of childhood. If children cough after exercising, they have asthma; if they have trouble reading, they are dyslexic; if they are unhappy, they are depressed; and if they alternate between unhappiness and liveliness, they have bipolar disorder. While these diagnoses may benefit the few with severe symptoms, one has to wonder about the effect on the many whose symptoms are mild, intermittent or transient.

The other source is the drive to find disease early. While diagnoses used to be reserved for serious illness, we now diagnose illness in people who have no symptoms at all, those with so-called predisease or those "at risk."