Chronic fatigue syndrome, also called ME/CFS, does not have a single known cause. The best-supported view is that it likely results from a mix of triggers and vulnerabilities, including infections, immune system changes, genetic predisposition, and problems with energy regulation in the body.

What researchers think may contribute

  • Infections: It often starts after a viral illness, such as Epstein-Barr virus, and has also been linked to COVID-19 and other infections.
  • Immune system changes: Some studies suggest abnormal immune activity or autoimmune-like patterns may play a role.
  • Genetics: Family clustering suggests inherited susceptibility in some people.
  • Energy metabolism issues: Researchers think cells may have trouble making or using energy normally.
  • Stress or physical trauma: These may trigger symptoms in some cases, though they are not considered the sole cause.

The short version

There is no single proven cause of chronic fatigue syndrome, and it probably develops from multiple factors acting together.

When to get checked

If fatigue lasts more than a few weeks, is severe, or comes with sleep problems, brain fog, pain, or symptoms that worsen after activity, it’s worth seeing a clinician to rule out other causes and discuss ME/CFS.

If you want, I can also give you a simple symptom list , how doctors diagnose it , or how ME/CFS differs from regular fatigue.