to add to sqads comments. osteo often affects a pair of joints eg both hips or both shoulders. Where it is assymetrical it is often use based (eg in joints of dominant hand) or springs from an injury which has caused the sufferer to use their body assymetrically, eg over use a good leg because of an injury to the other one.
Rheumatoid arthritis is one aspect of a whole collection of illnesses where the body actually attacks itself due to a malfunction of the immune system. It can affect only one joint but this is rare and tends to be a disease of flare up and remission. Stress. diet and over exertion are some of the factors that can trigger a flare up.
good info here
http://www.medicinene...d_arthritis/page2.htm
if you are offered a choice of the UK or US sites, choose US as the UK site for some reason is a lot less detailed.