What is fatigue? Fatigue is a feeling or sensation of tiredness, weariness, exhaustion, weakness or low energy that occurs during or after routine activities. Fatigue can also be a feeling of inadequate energy to begin such activities. Fatigue is a symptom of a wide variety of mild to serious diseases, disorders and conditions including infection, inflammation, trauma, malignancy, chronic diseases, autoimmune diseases, and mental illnesses. Fatigue can occur in any age group or population, but it is particularly common in the elderly and in people with chronic diseases. Depending on the cause, the sensation of fatigue can last briefly and disappear quickly, such as after exertion or a single night of insomnia. Fatigue can also be chronic and ongoing over a longer period of time, such as with fatigue due to chronic fatigue syndrome, depression, or heart failure. Fatigue can be a symptom of a serious or life-threatening condition. If you experience fatigue that is not alleviated with good sleep habits and adequate nutrition, seek prompt medical care and talk to your medical professional about your symptoms. If you, or someone you know, is experiencing fatigue with chest pain, difficulty breathing, dizziness, or a change in consciousness or alertness, seek immediate medical care (call 911).