Actually, "arthritis" is not a specific illness; it is a colloquial term for joint discomfort or disease. There are about 100 different forms of arthritis and associated disorders. Arthritis may and does affect people of all ages, genders, and ethnicities, and it is the biggest cause of disability in the United States. There are many types of arthritis, but they can be divided into two main groups: autoimmune and reactive (or inflammatory). Autoimmune arthritis is caused by your body's immune system attacking itself. The most common form of this type of arthritis is rheumatoid arthritis (RA). Reactive arthritis is just what it sounds like: Your body reacts to something else in your body - in this case, bacteria or a virus. This will usually result in joint pain and stiffness. Bacterial arthritis can also occur when there is direct infection of the joints by bacteria such as Streptococcus agalactiae (group B strep) or Staphylococcus aureus. Viral arthritis can occur with various forms of viral infections, such as chickenpox, measles, mumps, or influenza. The three main types of arthritis are osteoarthritis (OA), RA, and juvenile idiopathic arthritis (JIA). These three types of arthritis account for almost 95% of all cases of arthritis. Osteoarthritis is the most common type of arthritis.