More than three million Americans suffer from chronic fatigue syndrome, according to the CDC.

The syndrome is characterized by at least six months exhaustion not helped by bed rest. Patients also report brain fog after doing routine activities like work or school.

Chronic fatigue is estimated to cost Americans more than $18 billion a year.

There is no cure. Doctors have not been able to figure out a cause, but research suggests it is the body’s overreaction to an infection.