For most forms of arthritis, the exact cause is unknown. Infectious forms may be traced to a specific bacterial agent (such as gonorrhea) that invades a joint. Osteoarthritis (OA) is the result of the breakdown of cartilage, usually related to aging, but it is not fully understood why the degeneration begins. It may be the result of a malfunction in the cartilage and the metabolism of the underlying bone. Rheumatoid arthritis (RA), juvenile rheumatoid arthritis and psoriatic arthritis are autoimmune conditions, meaning the body's overactive immune system mistakenly attacks healthy tissues. The causes of autoimmune conditions are not well understood. Gout and pseudogout involve deposits of crystals in joints, a process that is only partly understood.

Elements called risk factors may increase the likelihood of developing a disease or condition. However, not everyone with the disease may exhibit risk factors, and not everyone with risk factors develops the disease. Some of the risk factors associated with forms of arthritis include:

• Age. OA, by far the most common form of arthritis, is more likely to develop as people get older. About 70 percent of people over age 70 show evidence of OA in x-ray images, according to the American College of Rheumatology. Advanced age is also a risk factor for pseudogout. However, RA, ankylosing spondylitis, Reiter's syndrome and often gout are more common in earlier adulthood, and children can also get arthritis.
  
  
• Weight. Excess weight may increase the likelihood of developing osteoarthritis. The extra stress placed on joints may contribute to the degeneration of cartilage.
  
  
• Sex. Women are more likely than men to develop many types of arthritis, including OA and especially RA. Several arthritic conditions are more common in men, including gout, Reiter's syndrome and ankylosing spondylitis.
  
  
• Personal medical history, including joint injury. People who have experienced acute joint trauma or overused certain joints in sports or in their work may develop arthritis. An earlier joint injury also makes it more susceptible to developing arthritis.

• Genetics. Studies show that a genetic marker called HLA-B27 can be found in some people with certain forms of arthritis, such as ankylosing spondylitis. Many people with RA have a gene called HLA-DR4, and other gene variants including STAT4 and TRAF1-C5 have also been linked to RA. Not every person with arthritis has these genes. People who do have the gene have a greater tendency to develop some forms of the disease. Genetic variants have also been linked to OA, and heredity may play a role in developing other forms of arthritis as well.
  
  
• Lifestyle. In addition to obesity, other lifestyle factors may contribute to certain forms of arthritis. For instance, gout may develop when excess uric acid accumulates in the joints. Uric acid comes from food or drink high in purines, so people with excess purines in their diet (e.g., alcohol, organ meats, anchovies) are more likely to develop gout.