Reactive arthritis. Reactive arthritis is a chronic form of arthritis that often occurs following an infection of the genital, urinary or gastrointestinal system. Features of reactive arthritis include inflammation and swelling of the joints (primarily the knee, sacroiliac joints and joints of the feet), eyes and structures within the gastrointestinal or genitourinary tracts, such as intestines, kidneys or bladder.

The National Institute of Arthritis and Musculoskeletal and Skin Diseases answers your questions about reactive arthritis.

Lyme disease.  Lyme disease is an infectious disease characterized by a skin, rash, joint swelling and flu-like symptoms. The disease is caused by the bite of a tick infected with a bacterium called B. burgdorferi. The knee is often the first joint affected by Lyme disease.

Learn about the effects of Lyme disease from the Arthritis Foundation Disease Center.

Lupus. Lupus, or systemic lupus erythematosus, is a chronic autoimmune disease, meaning the body's immune system creates antibodies that attack healthy tissues. In addition to causing inflammation in the knee and other joints, lupus can affect many organs, including the skin, heart, lungs, and kidney.

Learn more about lupus from the Arthritis Foundation Disease Center.

Ankylosing spondylitis. Ankylosing spondylitis is a form of arthritis that primarily affects the spine, causing inflammation in the spine that can lead to chronic pain and stiffening of the spine. In some people, particularly younger people, it begins with pain and swelling in the knee rather than the spine.

Learn more about ankylosing spondylitis from the National Library of Medicine.

Psoriatic arthritis. Psoriatic arthritis is a form of arthritis accompanied by the skin disease psoriasis. The skin disease often precedes the arthritis; in a small percentage the joint disease develops before the skin disease.

Learn about psoriatic arthritis from the Arthritis Foundation Disease Center.

Read more about psoriatic arthritis.

Infectious arthritis.  Also called septic arthritis, infectious arthritis refers to arthritis that is caused by an infection within the joint. Infectious arthritis is often cause by bacteria that spread through the bloodstream to the joint. Sometimes it is caused by viruses or fungi.

Learn more about septic arthritis from the National Library of Medicine.