While often referred to as if it were a single disease, arthritis is actually an umbrella term used for a group of more than 100 medical conditions that collectively affect nearly 70 million adults and 300,000 children in America alone.

While the most common form of arthritis - osteoarthritis - is most prevalent in people over 60, arthritis in its various forms can start as early as infancy. Some forms affect people in their young-adult years as they are beginning careers and families and still others start during the peak career and child-rearing years.

The common thread among these 100-plus conditions is that they all affect the musculoskeletal system and specifically the joints - where two or more bones meet.

Arthritis-related joint problems include pain, stiffness, inflammation and damage to joint cartilage (the tough, smooth tissue that covers the ends of the bones, enabling them to glide against one another) and surrounding structures. Such damage can lead to joint weakness, instability and visible deformities that, depending on the location of joint involvement, can interfere with the most basic daily tasks such as walking, climbing stairs, using a computer keyboard, cutting your food or brushing your teeth.

For many people with arthritis, however, joint involvement is not the extent of the problem. Many forms of arthritis are classified as systemic, meaning they can affect the whole body. In these diseases, arthritis can cause damage to virtually any bodily organ or system, including the heart, lungs, kidneys, blood vessels and skin. Arthritis-related conditions primarily affect the muscles and the bones.

Arthritis causes pain, loss of movement and sometimes swelling. Some types of arthritis are:

• Osteoarthritis, a degenerative joint disease in which the cartilage that covers the ends of bones in the joint deteriorates, causing pain and loss of movement as bone begins to rub against bone. It is the most prevalent form of arthritis.
• Rheumatoid arthritis, an autoimmune disease in which the joint lining becomes inflamed as part of the body's immune system activity. Rheumatoid arthritis is one of the most serious and disabling types, affecting mostly women.
• Gout, which affects mostly men. It is usually the result of a defect in body chemistry. This painful condition most often attacks small joints, especially the big toe. Fortunately, gout almost always can be completely controlled with medication and changes in diet.
• Ankylosing spondylitis, a type of arthritis that affects the spine. As a result of inflammation, the bones of the spine grow together.
• Juvenile arthritis, a general term for all types of arthritis that occur in children. Children may develop juvenile rheumatoid arthritis or childhood forms of lupus, ankylosing spondylitis or other types of arthritis.
• Systemic lupus erythematosus (lupus), a serious disorder that can inflame and damage joints and other connective tissues throughout the body.
• Scleroderma, a disease of the body's connective tissue that causes a thickening and hardening of the skin.
• Fibromyalgia, in which widespread pain affects the muscles and attachments to the bone. It affects mostly women.