Psoriatic arthritis (PA) can develop only in people who have psoriasis, a common skin disease. PA is more common in patients whose nails are affected by psoriasis. Both diseases are autoimmune disorders, which occur when a person's immune system is hyperactive and mistakenly attacks the body's own tissues.

This abnormal response in psoriasis patients causes excessive production of skin cells, which build up as rough and dry dead skin cells that become thick scales. PA develops in about 15 percent of psoriasis patients, with the joint inflammation caused by the immune system's overreaction to the skin disease, according to the American College of Rheumatology.

People who have a parent or sibling with PA are at greater risk of developing the disease. Researchers have found gene mutations that appear to be associated with PA. In some cases, fungal, viral or bacterial infection or physical trauma may trigger PA in people who have a genetic predisposition to the disease. Factors that may contribute to PA include:

• Skin injuries
• Reactions to medications or vaccines
• Infections, especially streptococcal (strep) infections
• Stress
• Alcohol and poor nutrition
• Overexposure to the sun
• Prolonged exposure to irritating chemicals such as disinfectants and paint thinners

Factors that increase the risk of developing PA include:

• Heredity. About half of the people with PA have a close relative who has the disease.
  
  
• Age. PA most commonly affects adults between the ages of 30 and 50.
  
  
• Race. Caucasians are most likely to develop PA.
  
  
• Sex. PA tends to affect men and women equally, although certain forms of the disease are more likely to affect men (DIP and spondylitis) or women (symmetric arthritis).
  
  
• Human immunodeficiency virus (HIV) infection. PA occurs more often in patients who are HIV-positive. In addition, HIV infection may exacerbate cases of psoriasis.