The five most common types of arthritis are:

Osteoarthritis. The more prevalent type of arthritis, osteoarthritis, also called degenerative joint disease, affects between 15 million and 20 million Americans, usually over the age of 45.  Under age 45, osteoarthritis is more common among men; over age 45, women are more often affected. The pain of osteoarthritis is often described as a deep ache, and can feel as if it is spreading to the muscles around the affected joint. Osteoarthritis tends to damage the larger, weight-bearing joints such as the knees and hips, but fingers and toes can be affected as well. Osteoarthritis occurs when the smooth lining of a joint, the cartilage, begins to flake and crack, causing the underlying bone to thicken and distort.  Classic symptoms of osteoarthritis are aching, stiffness and limited range of motion of the joint.  The reasons osteoarthritis develops are many. Repetitive motion, poor posture and incorrect walking, obesity, nutritional deficiencies and trauma all can play parts. Osteoarthritis is the leading cause of disability in the elderly.

Rheumatoid Arthritis. An inflammatory disease, rheumatoid arthritis (RA) occurs when the immune system malfunctions and turns on the body.  Joints affected by RA become extremely painful, stiff, swollen, red and warm to the touch. Joints hurt during rest and activity. Women, more than men, get rheumatoid arthritis, usually between the ages of 36 and 60.  Small joints, such as the fingers or toes, are most frequently affected. Wrist, knees, ankles, neck bones — even eyes, heart, lungs and blood vessels — also can come under attack.  The deformity and disability associated with the disease peaks in middle age, but the systemic involvement (heart, lungs, etc.) occurs in the elderly.  The cause of RA is still unclear, but it is believed to be due to a combination of genetic and environmental factors.

Ankylosing Spondylitis (Spinal Arthritis). About 300,000 Americans, most of them men, have this form of the disease, which causes immobility of the back and often the shoulders and neck. Ankylosing Spondylitis can make life miserable for the sufferer.  Severe back pain, in the middle back and often moving up to the neck, can make turning the head, bending or stooping very difficult.  Mornings can be particularly difficult, with spinal pain that continues for hours. The cause is unknown, but some research suggests spinal arthritis is triggered by exposure to bacteria, viruses or environmental factors.  About half the people with this form of arthritis also have arthritis in their hips and shoulders.

Gout.  Usually associated with lifestyle and diet, gout affects some 1 million persons, usually men.  Gout occurs when uric acid builds up and enters the tissues or joints. It appears to be triggered by alcohol, dehydration from excessive heat, smoking or by certain medications.  If left uncontrolled, gout can cause kidney disease, high blood pressure and joint deformity.  The area affected by gout becomes swollen, purplish red and acutely painful.

Systemic Lupus Erythematosus (SLE). Technically, SLE is not arthritis but an immune-related form of arthralgia.  It affects small and large joints. The 131,000 Americans who have SLE are most commonly younger women, ages 20 to 40. The disease can be difficult to diagnose; symptoms include severe fatigue, a "butterfly" rash across the face, debilitating pain and swelling in the hands, wrists, elbows, knees, ankles or feet, or morning stiffness in the joints.

Another common form of arthritis:

Juvenile Rheumatoid Arthritis (JRA).  The most prevalent form of arthritis in children, RJA is a disorder of the immune system that affects as many as 100,000 children in the United States alone. The three main types of JRA are pauciarticular JRA, which affects only a few joints and may cause eye inflammation; polyarticular JRA, which affects many joints, and systemic JRA, which affects many body parts, including internal organs and joints. While permanent damage to joints is less common in children than in adults, JRA can cause growth problems and joint inflammation and damage.  Early diagnosis and treatment is important to control inflammation, relieve pain, prevent and control joint damage, and help young patients maintain their functional abilities.

How Arthritis Happens

Arthritis, being many diseases, has many causes.  Gout, for example, is caused by an excess of uric acid (one of the body's waste products) in the joints, is triggered by alcohol, smoking, certain medications or excessive dryness.  The most common forms of arthritis are osteoarthritis and rheumatoid arthritis.

Osteoarthritis, while not an inevitable part of growing older, happens more frequently in older people. Some people describe the pain it brings as "like a rusty hinge," especially when a large joint like the hip or knee is involved. There may be a genetic component — you may inherit a tendency toward arthritis from your parents, or be born with faulty cartilage that breaks down prematurely.  Obesity can trigger osteoarthritis in the knee, or work- and sports-related injuries can lead to osteoarthritis.

The causes of rheumatoid arthritis aren't known, but researchers suspect that agents, perhaps viruses, trigger RA in people with an inherited tendency toward the disease. Food allergies may also trigger RA, and changing one's diet can sometimes improve RA symptoms, according to Michael Loes, M.D., of the Arizona Pain Institute. Many people with RA and its swollen, painful joints, research has found, have certain genetic markers that non-RA people don't have.