Sort of like, okay, if I have a hundred water balloons and I dropped fifty of those water balloons off the top of my house growing up, and fifty off the top of the Empire State building, would the latter always be that much worse? Just a few off the first and I can guess the outcome, and let's call that a control group, but the second, who knows... sure you might get lucky and knock one or two people out (or get a couple of good people showing results or seeming connections you want in a study) but you probably need more than a few to prove it isn't just a fluke. ;) Okay, messing around with the water balloons. Not with the study thing.
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]
https://www.thelancet.com/journals/laninf/article/PIIS1473-3...