Rheumatoid arthritis

Rheumatoid arthritis occurs when your immune system attacks the synovium — the lining of the membranes that surround your joints. The resulting inflammation thickens the synovium, which can eventually invade and destroy the cartilage and bone within the joint. The tendons and ligaments that hold the joint together weaken and stretch. Gradually, the joint loses its shape and alignment.

Doctors don't know what starts this process, although a genetic component appears likely. While your genes don't actually cause rheumatoid arthritis, they can make you more susceptible to environmental factors — such as infection with certain viruses and bacteria — that may trigger the disease.