Lyme disease is a bacterial illness which is primarily spread through insect bites, especially from deer ticks. Symptoms vary, depending on what stage the disease is in. In its early stages, Lyme disease has a number of vague symptoms, including fever, fatigue and muscle weakness; however, patients will develop a characteristic rash which aids a rapid diagnosis. In its later stages, Lyme disease can cause severe and chronic symptoms with permanent consequences, particularly for the heart and central nervous system. Arthritic symptoms may also result from late-stage infection, especially in the knees. Once diagnosed, Lyme disease usually responds well to treatment with antibiotics.