The cause of<unk>  is not well understood. The pain and stiffness result from the activity of inflammatory cells and proteins that are normally a part of the body's disease-fighting immune system, and the inflammatory activity seems to be concentrated in tissues surrounding the affected joints. During this disorder, the white blood cells in the body attack the lining of the joints, causing inflammation. Inherited factors also play a role in the probability that an individual will develop<unk> . Several theories have included viral stimulation of the immune system in genetically susceptible individuals. Infectious diseases may be a contributing factor. This would be expected with sudden onset of symptoms, for example. In addition, new cases often appear in cycles in the general population, implying a viral connection. Studies are inconclusive, but several somewhat common viruses were identified as possible triggers for<unk> . The viruses thought to be involved include the adenovirus, which causes respiratory infections; the human parvovirus B19, an infection that affects children; and the human parainfluenza virus. Some sufferers attribute the onset of<unk>  to stress. Persons having the HLA-DR4 type of human leucocyte antigen appear to have a higher risk of<unk> .
Polymyalgia rheumatica
Giant cell arteritis
Fibromyalgia
Myositis
Lupus erythematosus
Rheumatism
Dermatopolymyositis
Oligoarthritis