Reactive arthritis is a chronic form of arthritis which results in inflamed joints, inflammation of the eyes (conjunctivitis), and inflammation of the genital, urinary, or gastrointestinal systems. This form of joint inflammation is called "reactive arthritis" because it is felt to involve an immune system that is "reacting" to the presence of bacterial infections in the genital, urinary, or gastrointestinal systems. Hence, certain people's immune systems are genetically primed to react aberrantly when these areas are exposed to certain bacteria. The aberrant reaction of the immune system leads to spontaneous inflammation in the joints and eyes. This can be perplexing to the patient and the doctor when the infection has long passed at the time of appearance with arthritis or eye inflammation.

Reactive arthritis has been referred to as Reiter syndrome also in the past. Reactive arthritis most often occurs in patients in their 30s or 40s, but it can occur at any age. The form of reactive arthritis that occurs after genital infection (venereal) occurs more frequently in males. The form that develops after bowel infection (dysentery) occurs in equal frequency in males and females. Reactive arthritis is considered a systemic rheumatic disease. This means it can affect other organs than the joints, causing inflammation in tissues such as the eyes, mouth, skin, kidneys, heart, and lungs.

Reactive arthritis shares many features with several other arthritic conditions, such as psoriatic arthritis, ankylosing spondylitis, and arthritis linked with Crohn's disease and ulcerative colitis. Each of these arthritic conditions can cause similar disease and inflammation in the spine and other joints, eyes, skin, mouth, and various organs. In view of their similarities and tendency to inflame the spine, these circumstances are jointly referred to as "spondyloarthropathies."

Overall, men between the ages of 20 and 40 are most likely to develop reactive arthritis at any stages. Nevertheless, evidence shows that although men are nine times more likely than women to develop reactive arthritis due to many acquired infections, women and men are equally likely to develop reactive arthritis as a result of food-borne infections. Women with reactive arthritis often have milder symptoms than men.

Less common symptoms are mouth ulcers and skin rashes. Any of these symptoms may be so mild that patients do not notice them. They usually come and go over a period of several weeks to several months. Reactive arthritis often affects the urogenital tract, including the prostate or urethra in men and urethra, uterus, or vagina in women. Men may notice an increased need to urinate, a burning sensation when urinating, penis pain and a fluid discharge from the penis. Some men with reactive arthritis develop prostatitis. Symptoms of prostatitis can include fever and chills, as well as an increased need to urinate and a burning sensation when urinating.

Women with reactive arthritis may develop problems in the urogenital tract, such as cervicitis or urethritis, which can cause a burning sensation during urination. In addition, some women also develop salpingitis or vulvovaginitis. These conditions may or may not cause any arthritic symptoms.