The association with allergies also serves to imply a genetic predisposition for the illness, and this can help to predict who may be at risk for developing CFIDS.

Multiple allergies are perhaps the most obvious sign of immune system overreaction. Patients with CFIDS frequently have a past history of allergies, implying that their immune response is genetically primed for a vigorous response. Other patients with CFIDS develop allergic symptoms after the onset of their illness. CFIDS patients have a hair trigger for allergies.

Many patients with cancer are depressed, yet depression is not the cause of cancer.