Lyme disease is a multi-system infectious condition that frequently causes fatigue and headache and can progress to arthritis, memory problems and other complications. It is the leading tick-borne disease in the United States.

Lyme disease occurs most often in rural and suburban areas in the U.S. Northeast, mid-Atlantic region and upper Midwest. It also affects parts of Canada, Europe, Australia and Asia, where it is often called Lyme borreliosis. Lyme disease was first identified in the United States in 1975 near Lyme, Conn. However, its existence has been documented in Europe for almost a century.

Lyme disease progresses in three stages: early localized, early disseminated and late Lyme disease. As the bacteria spread, symptoms of fatigue and headache may appear.

If the disease progresses to late Lyme disease, arthritic symptoms can appear, along with cognitive difficulties and skin puffiness. These may take months or years to appear.

When treated, most people recover quickly and experience very few complications from Lyme disease. A small percentage of patients recover slowly or do not fully recover from symptoms if they are not treated in a timely manner.

According to the U.S. Centers for Disease Control and Prevention (CDC), 19,931 cases of Lyme disease were reported from 46 states and the District of Columbia in 2006. The incidence has generally risen since 1991, when fewer than 10,000 cases were reported, but might have peaked in 2002. The vast majority of U.S. cases occur in the region stretching from Maine to Virginia and in Minnesota and Wisconsin. Parts of northern California, Iowa, Michigan and northern Illinois also have increased risk. The agency considers the Southeast and southern Midwest to be low-risk areas.

About eight cases for every 100,000 Americans were reported in 2006, according to the CDC. In the 10 states where Lyme disease is most common, there were about 30 cases for every 100,000 people.

Bacteria called Borrelia burgdorferi cause Lyme disease. These bacteria are transmitted to humans through bites from infected ixodes ticks, which may be referred to as deer ticks or black-legged ticks in the eastern United States and western black-legged ticks in the western United States. These tiny ticks are smaller than those typically found on a dog and are difficult to spot.

The disease spreads when an infected tick takes a blood meal, passing infectious bacteria to an animal (usually a deer or mouse, sometimes another mammal or a bird) and making them a host for the disease. Other ticks feeding on the infected host animal contract the disease themselves, then pass it to more deer or mice. Ticks await new hosts by hiding within areas dense with grass or shrubs, most notably those along boundaries between residential properties and surrounding forest land and along deer paths. A tick senses carbon dioxide exhaled by humans or animals as they pass. The tick attaches itself to body parts that happen to rub up against it.

The tick must remain affixed to human skin for at least 36 hours for humans to become infected.  Furthermore, the tick can take as long as 24 hours from the time it attaches itself before feeding may even begin.  As a result, Lyme disease affects very few people who are bitten, even in areas with high populations of infected ticks. 

Lyme disease is most common in the summer and early fall when biting ticks are young, small and difficult to detect. Less frequent are cases in the late fall, early winter and early spring when biting ticks are fully developed and easier to spot.

The ticks that transmit Lyme disease can also spread two other conditions: human granulocytic anaplasmosis (HGA) and babesiosis. Infection with HGA (formerly known as ehrlichiosis) may be widespread but unrecognized, according to the CDC. Symptoms include malaise, fever, muscle pain (myalgia) and headache. In rare cases death has resulted.

Babesiosis is an infection of the red blood cells by a tiny parasitic protozoan. It can cause anemia and a malaria-like illness marked by fever, chills, excessive perspiration, muscle pain, headache and malaise. Patients infected with both diseases are more likely than those afflicted only with Lyme disease to experience symptoms including fatigue, anorexia (loss of appetite), unstable emotions, nausea, conjunctivitis (an inflammation of the eye) and splenomegaly (enlarged spleen), according to the National Institutes of Health (NIH). In rare cases death has resulted. Babesiosis may affect about 10 percent of Lyme disease patients in southern New England, according to the NIH.

As with Lyme disease, HGA and babesiosis are treated with antibiotics and respond best to early treatment.