There are two main kinds of arthritis: osteoarthritis and rheumatoid arthritis. They have different causes and effects on the body. As the most common kind of arthritis, osteoarthritis often occurs as wear and tear as we age. It happens when our joints’ cartilage wears down, which is gradual over time, or hastened by a joint injury or infection. With rheumatoid arthritis, the body’s immune system attacks the lining of a joint capsule which then becomes inflamed. This process can eventually destroy cartilage and bone within the joint.