4. Infections
There are many recorded cases of people who develop a persistent condition of fibromyalgia after an infection. It is still unclear whether infection can cause fibromyalgia to appear in the first place, or if there was an asymptomatic underlying condition,
like mild fibromyalgia or at least a predisposition due to genetic and/or historical factors, which got worse after the infection. Many doctors believe there is an autoimmune component to fibromyalgia, so it does make sense to think that any event that triggers the immune system might also awake this condition.
In many cases, the beginning of fibromyalgia goes undetected, because many of its symptoms are very similar to the flu (pain, fatigue, bowel problems), and people only seem to realize something strange is going on when they see their “flu” isn’t going away after a reasonably long time. The immune hypothesis seems to be the stronger possible explanation for this.