Just about any joint can be affected by Lyme arthritis. Although the knee is by far the most common, other commonly affected joints include the ankle, elbow and shoulder.

The location of the tick bite does not determine which joints are affected. In fact, most tick bites are not near the knee.

It's thought that arthritis occurs in Lyme disease because the bacterium that causes Lyme disease (called B. burgdorferi) travels from the tick into the bloodstream and is deposited in the joint where the body's immune response causes inflammation.

It's not clear why the knee if the favored joint or why only one or a few joints are involved in most cases. It's also unclear why up to 40% of people with untreated Lyme infection never get arthritis at all.