One in three adult Americans suffer from some form of arthritis and the disease affects about twice as many women as men.

Arthritic diseases include rheumatoid arthritis and psoriatic arthritis, which are autoimmune diseases; septic arthritis, caused by joint infection; and the more common osteoarthritis, or degenerative joint disease. Arthritis can be caused from strains and injuries caused by repetitive motion, sports, overexertion, and falls. Unlike the autoimmune diseases, osteoarthritis largely affects older people and results from the degeneration of joint cartilage. Other forms are discussed below.

Arthritic joints can be sensitive to weather changes. The increased sensitivity is thought to be caused by the affected joints developing extra nerve endings in an attempt to protect the joint from further damage.

Signs and symptoms
All arthritides feature pain, which is generally worse in the morning and on initiating movement, and resolves in the course of time. In elderly people and children, the pain may not be the main feature, and the patient simply moves less (elderly) or refuse to use the affected limb (children).

When faced with joint pain, a doctor will generally ask about several other medical symptoms (such as fever, skin symptoms, breathlessness, Raynaud's phenomenon) that may narrow down the differential diagnosis to a few items, for which testing can be done.

Arthritis and fever together are pointers towards septic arthritis (see below). This is a medical emergency, and requires urgent referral to a rheumatologist.

Diagnosis
The various types of arthritis can be distinguished by the pace of onset, the age and sex of the patient, the amount of (and which) joints affected, additional symptoms (such as psoriasis, iridocyclitis, Raynaud's phenomenon, and rheumatoid nodules), and other clues.

Blood tests and X-rays of the affected joints are often performed to make the diagnosis. X-rays can show erosions or bone appositions.

Screening blood tests: full blood count, electrolytes, renal function, liver enzymes, calcium, phosphate, protein electrophoresis, C-reactive protein and the erythrocyte sedimentation rate (ESR). Specific tests are the rheumatoid factor, antinuclear factor (ANF), extractable nuclear antigen and specific antibodies whenever the ANF is found to be positive.

Treatment
Treatment options vary depending on the precise condition, but include surgery, and drug treatment, reduction of joint stress, physical and occupational therapy, and pain management. There are also numerous herbal remedies that purportedly treat arthritis, including Harpagophytum procumbens. For specifics, see the articles on the individual conditions listed below.

In March 2005, researchers at Harvard Medical School and Brigham and Women's Hospital in the USA found that a diet rich in oily fish raised the body's production of an anti-inflammatory fat, and may thus reduce the effects of arthritis. According to their study published in the Journal of Experimental Medicine, this diet worked best when combined with low aspirin doses.