Arthritis is a group of conditions that affect the health of the bone joints in the body. Arthritic diseases include rheumatoid arthritis and psoriatic arthritis, which are autoimmune diseases; septic arthritis, caused by joint infection; and the more common osteoarthritis, or degenerative joint disease. Arthritis can be caused from strains and injuries caused by repetitive motion, overexertion, and falls. Unlike the autoimmune diseases, osteoarthritis largely affects older pets and results from the degeneration of joint cartilage. Arthritic joints can be sensitive to weather changes. The increased sensitivity is thought to be caused by the affected joints developing extra nerve endings in an attempt to protect the joint from further damage.