These conditions have varying genetic and environmental causes, reports The Washington Post.
"This is a very important step forward in the field of chronic fatigue syndrome research," said Julie Gerberding with the Centers for Disease Control and Prevention in Atlanta. The CDC sponsored the study that involved intense medical and psychological tests of patients.
The mysterious ailment manifests in many forms. But the new study says their common feature is that the brain and immune system of patients do not respond normally to physical and psychological stresses. The findings are published in the April issue of the journal Pharmacogenomics.
The researchers said continued clarification of the precise genes and hormones involved will lead to better diagnostic tests and therapies for the ailment, The Post said.