Illness usually begins in the spring or summer and is typically divided into three different stages, often with apparent recovery between stages. The different stages may overlap but can occur independently. It is caused by a specialized type of bacteria called a spirochete. It is usually transmitted by the bite of an infected tick or flea, although other insects which feed on animal blood may be involved. Except for the fatigue and lethargy, which are often constant and can be incapacitating, the symptoms tend to be intermittent and changing.
Fever is usually of a low grade, but sometimes becomes high, especially in children.
This stage of the disease may last for months with one set of symptoms predominating for a while only to change to another very different manifestation. The small red area around the insect bite begins to expand. The generally circular area may be as large as 14 inches, but often is smaller than 1 inch. Sometimes the center remains intensely red and hard, occasionally even forming blisters which later burst and form a heavy dark scab. This type lesion could easily be mistaken for a spider bite.