Reactive arthritis

Reactive arthritis (ReA) is an autoimmune condition that develops in response to an infection in another part of your body. Coming into contact with bacteria and developing an infection can trigger reactive arthritis. Reactive arthritis has symptoms similar to arthritis or rheumatism. It is caused by another disease, and is thus "reactive", i.e., dependent on the other condition. The "trigger" infection is typically missing in the chronic case.
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Reactive arthritis Another name for Reiter's syndrome. Mentioned in: Reiter's Syndrome reactive arthritis Reiter syndrome, see there [more]Reactive arthritis - Community and Resources

Reiter’s syndrome is also called "reactive arthritis" since it is thought to involve the immune system which is "reacting" to the presence of bacterial infections in the genital, urinary, or gastrointestinal systems. Accordingly, 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 inflammation in the joints and eyes.

noun
painful inflammation of a joint
reactive arthritis = arthritis caused by a reaction to something
rheumatoid arthritis = general painful disabling collagen disease affecting any joint, but especially the hands, feet, and hips, making them swollen and inflamed
see also OSTEOARTHRITIS