Conditions that can cause an uninjured knee to become swollen include pseudogout, gout, rheumatoid arthritis and bursitis, according to Arthritis-Health. Other conditions that cause a swollen knee are osteoarthritis and Baker's cyst.
Osteoarthritis is a condition where the cartilage between the joints in the knee start to break down over time, according to Arthritis-Health. This breakdown can cause the body to overproduce joint fluid, which causes the knee to swell.
Bursitis is caused when the bursae become inflamed, says Arthritis-Health. Bursae are little sacs that help lubricate the joint. When they're inflamed, they become filled with too much fluid, which also causes knee swelling. A bursa can also become infected and fill up with pus. In this case, the condition is called septic bursitis. The knee is not only swollen, but red and warm to the touch. A person who suspects septic bursitis should see her doctor as soon as possible.
Gout comes about because crystals of uric acid accumulate in the joint, says Arthritis-Health. The knee swells and is very painful, hot and red. Pseudogout is similar, though the crystals are not of uric acid but calcium pyrophosphate. Unlike real gout, which usually affects other joints, pseudogout mostly affects the knee.
Rheumatoid arthritis is actually an autoimmune disease where the body's own immune system attacks it, states Arthritis-Health. In this case, the immune system attacks the patient's joints, including the knee. The fluid-filled Baker's cyst is found at the back of the knee, says Mayo Clinic. It's also called a popliteal cyst.