wikipedia:

Reactive arthritis, previously known as Reiter's syndrome,[1] is a form of inflammatory arthritis[2] that develops in response to an infection in another part of the body (cross-reactivity). Coming into contact with bacteria and developing an infection can trigger the disease.[3] By the time a person presents with symptoms, the "trigger" infection has often been cured or is in remission in chronic cases, thus making determination of the initial cause difficult.

The most common triggers are intestinal infections (with Salmonella, Shigella or Campylobacter) and sexually transmitted infections (with Chlamydia trachomatis);[8] however, it also can happen after group A streptococcal infections.[9][10]