Bob politely interrupted the class I was teaching on tick-borne illnesses. 'Could this be what you're talking about?' he asked.

Standing, he pulled up the leg of his shorts to reveal a well-defined reddish spot on his upper thigh, about three inches at its widest, darker toward the outer edge.

'I've got a couple more,' he said, and proceeded to expose those to the rest of the class. 'They've been getting bigger,' he added.

Bob denied any knowledge of being tick bitten, but it looked like erythema migrans to me, the characteristic rash that announces Lyme disease in somewhere between 60 and 80 percent of patients. The rash may develop where the tick bit or anywhere else on the body, with a tendency to show up most often on thigh, groin, armpit and sometimes behind the knee. After it first appears, the rash grows, or 'migrates.' If erythema migrans emerges in the first week of the disease, the spots rarely grow to more than 2.5 inches across. Spots appearing two to four weeks later may reach eight inches across. The red rash typically feels warm and may cause a burning sensation, but seldom produces pain or itching.

Any more indications of illness? I inquired. 'Something like the flu,' Bob said, describing more of the early symptoms of Lyme disease-low-grade fever, headaches, unusual fatigue and muscle aches.

Infecting more than 15,000 North Americans annually, Lyme disease is the leading disease borne by carrier organisms, or vectors, in the U.S. Yet the illness remains widely overlooked. Often misdiagnosed or left untreated, it may resolve itself in the first month, but about half the cases will progress to more serious levels. Dubbed 'the great imitator,' Lyme disease mimics many common ailments. Physicians can easily draw the wrong conclusion about an infected patient's 'obvious' symptoms, ruling out Lyme disease and treating one of its imitated illnesses instead.

Early symptoms are often mild enough to be ignored. The rash doesn't always appear. And the ticks can go unnoticed, typically attaching themselves during the nymphal stage between May and August, when they take up less room than a poppy seed on your skin. 'Many of the worst cases of Lyme disease were contracted a decade ago when people were not as aware of the illness,' says David L. Weld, executive director of the American Lyme Disease Foundation. But missing an early diagnosis continues to be a grave problem, leading to debilitating headaches, neurological or cardiological complications and Lyme arthritis, which attacks the larger joints, particularly the knee.