With osteoarthritis, its mildest form, it is viewed as an inevitable consequence of old age, resulting in creaking joints from the wearing away of the cartilage. Rheumatoid arthritis strikes younger people, often between the ages of 20 and 40, and affects twice as many women as men. It mainly affects the joints, but can spread to other parts of the body. The synovial membrane around the joints becomes inflamed, which attracts more joint fluid to ease it. Eventually, the joint becomes swollen, stiff and warm because of an increased blood flow.Despite its prevalence, medicine is at a loss to understand the cause. In the early part of the century, scientists were convinced it was a chronic infection of the joints. This has given way to several new theories. The most persistent is that arthritis is hereditary, or that some people have a genetic tendency to develop the condition. Others believe it to be caused by a micro organism, mycoplasma, while a third school of thought maintains it is a malfunction of the body's metabolic and immune system. All agree, though, that they don't actually have the slightest idea.

Of the three, the theory that arthritis is a malfunction of the immune system is the most favoured, and most actively researched in the laboratories.