The gut microbiome may be a major factor for Chronic Fatigue Syndrome:
/www.washingtonpost.com/news/to-your-health/wp/2016/06/30/new-study-shows-chronic-fatigue-isnt-just-in-your-head-it-may-have-to-do-with-your-gut/?hpid=hp_hp-more-top-stories-2_fatigue-905a-stream%3Ahomepage%2FstoryCornell University researchers looked at stool and blood samples of 48 people diagnosed with chronic fatigue syndrome (or more formally, myalgic encephalomyelitis) and at 39 healthy volunteers.
They found two main differences: Through DNA sequencing, they found the stool samples in the patients with the condition had less diversity in bacteria present in the gut and that there were fewer that were anti-inflammatory. The blood samples were also distinct: There were markers of inflammation that the researchers theorized may be due to a "leaky gut from intestinal problems that allow bacteria to enter the blood."