Polymyalgia rheumatica is an arthritic syndrome that causes your muscles to feel achy and stiff due to mild inflammation in your joints and surrounding tissues. Most of the inflammation occurs in the hip and shoulder joints, but it may develop elsewhere in your body as well. In general, the inflammation isn't as severe as that in inflammatory types of arthritis, such as rheumatoid arthritis.
In polymyalgia rheumatica, inflammation occurs when white blood cells — which normally protect your body from invading viruses and bacteria — attack the lining of your joints (synovium). Researchers aren't sure what causes this abnormal immune system response, but they suspect that as with many disorders, both genetic and environmental factors are involved.
There may be a link between polymyalgia rheumatica and certain viruses, such as adenovirus, which causes respiratory infections ranging from the common cold to pneumonia; human parvovirus B19, the source of an infection that primarily affects children; and human parainfluenza virus.