Reactive arthritis is a type of arthritis that involves the immune system, which is "reacting" to the presence of bacterial infections in the body. It is a chronic form of the disease and it manifests itself in three conditions. First, the presence of swollen joints, then the swelling of the eyes, called conjunctivitis, and finally, the inflammation of the gastrointestinal, the genital and the urinary systems.

Reactive arthritis occurs when the immune system is exposed to a certain bacteria. Some people have immune systems that behave erratically after coming in contact with some forms of bacteria. The erratic behavior of the immune system causes the inflammation of the eyes and joints.

Not all people are vulnerable to reactive arthritis. Genetics play a significant role in the contraction of this disease. The latent reactive arthritis gene can be triggered by external factors, such as a bacterial infection.

However, the symptoms of reactive arthritis can be categorized into two groups: those affecting joints and those that do not.

This disease can cause joints, such as the wrists, knees, ankles and feet, to become inflamed. The affected joints are usually located in one side of the body, but not in both. The symptoms also include the classic manifestations of common arthritis, like joint pain, stiffness, redness and warmth. The fingers of patients are also usually swollen, causing the appearance of "sausage digits." The spine and neck can also become inflamed and may become painful and stiff.

The tendons, which connect muscles to bones, can also become swollen and may be painful when moved.

Restrictive arthritis also hits non-joint areas such as the eyes, urinary tract, skin, genitals, mouth, large intestine and even the aorta. These areas can become irritated and may cause pain and discomfort. There can be an irregular swelling of the iris and the white area of the eye, called iritis and conjunctivitis, respectively. Iritis can be extremely painful and it causes the patient to become visually sensitive to bright lights. On the other hand, conjunctivitis does not cause pain.

The urinary tract, which includes the urethra, the bladder and the prostate, can also be affected by this disease. Urethritis, or the infection of the urethra, is characterized by a burning sensation during urination and the excretion of pus from the penis. The penis itself could also be inflamed. Both the bladder and prostate too may become inflamed.

The skin is also affected. Small liquid-filled blisters can appear of the skin of the patient, particularly on the palms and soles. Cold sores, which are open and painless, may also appear on the patient's mouth, specifically on its linings and on the hard palate.

Diarrhea can also occur, due to the inflammation of the large intestine. Pus or blood may be present in the stool.

This disease was formerly called Reiter's Syndrome or Reiter's Disease, in honor of German military physician Hans Reiter. However, the disease's name was later changed because of Reiter's connection with the Nazi and his infamous experiments in Nazi camps.