Fatigue is a frequent symptom of many different viral infections. It is the body's natural response to a viral infection. Fatigue usually subsides after the body has dealt with the infection. Post-viral fatigue occurs when the fatigue caused by a viral infection lasts longer than the infection itself. The most noticeable symptom of post-viral fatigue is a severe lack of energy, which is not relieved by sleep. Headaches and heavy, aching muscles are also possible symptoms. These symptoms fluctuate, so a person may feel better one day and be able to participate in activities the next due to debilitating symptoms.