Various infections can cause an arthritis to develop 1 to 4 weeks later. The most common are sexually-transmitted Chlamydia, and various bacteria that cause diarrhea. It’s not the germ itself causing damage in the joint, but rather some sort of immune reaction that’s not understood.
Reactive arthritis usually involves the knee, or other large joints, but some people have it in the hands. There may also be pain around the heels, and a single entirely-swollen finger or toe called a “sausage digit” (“digit” in medicine means finger or toe, the formal term is “dactylitis”). Some people also have eye irritation, painful urination, or mouth ulcers.
The diagnosis is made by symptoms and examination alone. There are no blood tests or x-rays. If chlamydia was responsible, it might be found in the urine (or in the eye if it’s irritated), and should be treated. Blood tests might be done to rule out other causes of arthritis.
There’s no real treatment except for anti-inflammatory medication like NSAIDs to ease pain. The disease goes away on its own in a few months. For the few patients in whom it becomes chronic (over 6 months), rheumatologists have a variety of drugs to try.
A form of reactive arthritis with swollen joint, painful urination, and irritated eyes used to be called “Reiter Syndrome.” The name was dropped, because Reiter was a big Nazi in WWII.