What causes athritis?

Many: Many things can cause it, it could be that the cartilage is wearing away, a lack of fluid, autoimmunity (your body attacking itself), infection, or a combination of these things and others. Also depends on the type of arthritis.

Arthritis: Form of joint disorder that involves inflammation of one or more joints. There are over 100 different forms of arthritis. The most common form, osteoarthritis (degenerative joint disease), is a result of trauma to the joint, infection of the joint, or age. Other arthritis forms are rheumatoid arthritis, psoriatic arthritis, and related autoimmune diseases. Septic arthritis is caused by infection.