Chronic fatigue syndrome (CFS) is a debilitating illness that can contribute to depression, social isolation, and decreased productivity. The Mayo Clinic describes CFS as a disorder characterized by extreme fatigue that can't be explained by any underlying medical condition. The fatigue may worsen with physical or mental activity, but doesn't improve with rest. 8 Signs of Chronic Fatigue Syndrome: Loss of memory or concentration Sore throat Joint pain Tender or enlarged lymph