What are diseases and conditions that can cause knee pain, and what is the treatment for this knee pain?

Pain can occur in the knee from diseases or conditions that involve the knee joint, the soft tissues and bones surrounding the knee, or the nerves that supply sensation to the knee area. In fact, the knee joint is the most commonly involved joint in rheumatic diseases, immune diseases that affect various tissues of the body including the joints to cause arthritis.
Arthritis is inflammation within a joint. The causes of knee joint inflammation range from noninflammatory types of arthritis such as osteoarthritis, which is a degeneration of the cartilage of the knee, to inflammatory types of arthritis (such as rheumatoid arthritis or gout). Treatment of the arthritis is directed according to the nature of the specific type of arthritis. Many people suffer from arthritis; the pain and discomfort can be so limiting that some patients may require a total knee joint replacement. Knee replacement surgery often allows the patient to regain much of their mobility.
Swelling of the knee joint from arthritis can lead to a localized collection of fluid accumulating in a cyst behind the knee. This is referred to as a Baker cyst and is a common cause of pain at the back of the knee.
Infections of the bone or joint can rarely be a serious cause of knee pain and have associated signs of infection including fever, extreme heat, warmth of the joint, chills of the body, and may be associated with puncture wounds in the area around the knee. These infections are often diagnosed by aspirating joint fluid accumulations with a needle (joint aspiration) and examining the fluid microscopically and with microbial culture techniques. Treatment involves antibiotics and sometimes joint surgery.