The cause of arthritis is not fully known. It is thought that some people who are genetically predisposed develop the disease after they are exposed to environmental triggers. Other forms of arthritis can be caused by problems with the immune system or by a metabolic condition, such as gout.

Environmental factors that may contribute to the development of osteoarthritis include:

• obesity, which puts added strain on joints
• activities that involve repetitive movements of a particular joint, or
• previous damage to a joint, such as from a sports injury

People with a family history of osteoarthritis seem to be more prone to developing the condition, although the reason why has not been fully established. You are also more likely to develop arthritis if you smoke and if you don't do enough physical activity.

Most forms of arthritis are thought to be caused by a fault in the immune system that causes the body to attack its own tissues in the joints. This may be inherited genetically.

Arthritis caused by an infection is called 'reactive arthritis'. It's very difficult to diagnose and can develop at any age, but is more commonly seen in younger people. Reactive arthritis can last between a few weeks and 6 months.