According to his doctor, Rich David is a healthy 32-year-old man. Yet for years, David has believed otherwise. All it takes is a swollen gland or an upset stomach to set him off. Immediately, he assumes -- he knows -- that he's fatally ill.

"I'll waste days researching gruesome cancers on the Internet," he says. He can't concentrate on his work. He's so anxious that he can't eat; the resulting weight loss further terrifies him. Despite its comic reputation, hypochondria is a real psychiatric disorder, as real as depression or anxiety. And its effects can be devastating.

Hypochondria -- the conviction that one is ill, despite all evidence to the contrary -- affects as much as 5% of the U.S. population, according to the American Psychological Association. It often starts in a person's 20s and can be triggered by a medical scare or the illness of a friend or relative. It then can wax and wane over a person's life, flaring up during stressful times. It affects men and women equally.

"Hypochondriacs get caught in a cycle," says Arthur J. Barsky, MD, professor of psychiatry at Harvard Medical School and author of Stop Being Your Symptoms and Start Being Yourself. "The more they worry about a symptom, the worse it gets." They're often highly attuned to bodily sensations that most people ignore. Every ache, every cough, every stomach gurgle is evidence of something going catastrophically wrong.

Hypochondriacs don't just dwell on their disease, they act. They scour the Internet for information, earning some the moniker "cyberchondriacs." They demand lab tests from irritated doctors. They talk about it relentlessly.

Many of them can even admit that their fears don't make sense. In fact, the symptoms associated with hypochondria are not under the person's voluntary control. "I know I'm a hypochondriac," says David. "But when I get obsessed with a symptom, I can't shake the feeling that this time I really am sick."

Some experts compare hypochondria with anxiety disorders, especially obsessive-compulsive disorder. Just as someone with OCD has to check that the lights are off a dozen times, the hypochondriac can't resist researching and checking his symptoms.