"Doctor, bugs are crawling in my skin, sending out fibers, releasing secretions! Look at these awful sores—something's alive in them."

The plea may sound straight out of a sci-fi movie, but for some people, it's a sensation they face every day. And it's an experience they share with me.

As a tropical medicine specialist, my toughest challenge is not what you might imagine—a returning traveler afflicted with malaria, amebiasis, or dengue fever. Those are serious illnesses, but I know how to diagnose and treat them.

Instead, the patients who haunt me are the ones who believe they are infested with parasites, and—according to everything I know—are not. For decades, dermatologists, psychiatrists, and other specialists have called their condition delusional parasitosis. Today some sufferers are pursuing a new label: Morgellons disease. Not long ago, I saw my first case.

It began with an urgent request from a colleague of mine that I see his friend from church. From what he told me, Margo Riley was truly one of God's saints. For some time, she had been going to Mexico every few months to volunteer at rural clinics and orphanages.

Over the past year, however, Margo had taken no overseas trips and rarely even left her house. Like a modern-day Job, she suffered recurring nodules, pustules, and weeping sores on her skin. The affliction had turned her into a virtual recluse.

When she walked into my exam room, her misery was obvious. Her skin was blotched and marred. Her sagging posture and dull gaze echoed her despair. Then, as if fed by some new wellspring of energy, Margo's body cues changed.

"I don't know if Joe told you, but I have Morgellons disease," she said as she seated herself next to my desk. "I've read all about it on the Internet, and I'm sure I'm right."

By coincidence, I had also recently read about Morgellons, a syndrome whose victims suffer rashes, itching, burning skin, and recurrent skin ulcers studded with filaments and granules. In 2002 a former electron microscopist, searching for the cause of her son's rash, christened the blight after finding the name in a historical reference online. At the same time she started the Morgellons Research Foundation, which, according to its Web site, "is dedicated to finding the cause of an emerging infectious disease which mimics scabies and lice."

Skin eruptions on the back of a patient who says she has Morgellons (left), a condition unknown to most dermatologists. A different patient (right), whose back is covered with skin lesions resulting from her own scratching.

When Margo first announced she had Morgellons disease, her defiant tone was unmistakable. I let the challenge pass, however. Nothing would be gained if we started wrangling over her diagnosis before I had even reviewed the facts of her case.

"Please, just tell me what happened to you," I urged. "How did this start?"

"OK," she agreed, relaxing a little. "Last year our church group went to the Yucatán Peninsula. The place was loaded with bugs, so we all got bites. At first, mine just itched. Then they oozed, crusted, and spread."

So far, her problem sounded like a nasty but familiar tropical woe—bacterial impetigo. There's something about heat and humidity that fuels superficial skin infections.

"Did you take an antibiotic?"

"Yes, of course," she replied with a little more brio. "When I got back from Mexico, my doctor put me on Keflex and prednisone for the itching. After they didn't work, I tried so many other things I lost count. One specialist even treated me for fungus. Through it all, I still had scabs and sores—plus weird things coming out of me."

I needed to look more closely at Margo's skin. Although I couldn't place it, there was something unnatural about the scars on her face and arms. Then she hiked up the legs of her loose cotton pants. What I saw straightaway was the rough, angry scatter of infected hair follicles. Then I noticed some larger nodules on her thighs that looked chronically inflamed, as if they had been repeatedly unroofed while healing.