Viral infections. Because some people develop chronic fatigue syndrome after having a viral infection, researchers question whether some viruses might trigger the disorder.
Suspicious viruses include Epstein-Barr virus, human herpes virus 6 and mouse leukemia viruses. No conclusive link has yet been found.
Immune system problems. The immune systems of people who have chronic fatigue syndrome appear to be impaired slightly, but it’s unclear if this impairment is enough to actually cause the disorder.
Hormonal imbalances. People who have chronic fatigue syndrome also sometimes experience abnormal blood levels of hormones produced in the hypothalamus, pituitary glands or adrenal glands. But the significance of these abnormalities is still unknown.