What is weakness? Weakness is a feeling of being tired or exhausted, or experiencing a loss of strength. Weakness may not always be accompanied by obvious or visible illness. Short-term weakness may occur because of overwork, stress, or lack of sleep. You may also feel weakness after overcoming an illness, such as a cold or the flu. Some weakness may occur after vigorous physical activity. Weakness may occur throughout your entire body or in a specific area, such as your arms or legs. Weakness may also be localized to a single muscle such as a calf muscle in your leg. You may also feel weakness as a symptom of depression. Depression is defined as feeling blue, miserable or sad. While occasional periods of sadness are normal, long-term depression, called clinical depression, may indicate serious emotional or psychological problems. Weakness or fatigue that is persistent always requires the prompt attention of your health care provider. Weakness may also occur because of physical diseases or toxic disorders. Long-term (chronic) conditions, such as multiple sclerosis or an underactive thyroid, may cause weakness. Short-term (acute) conditions, such as a pinched nerve or a urinary tract infection, may also cause weakness. Other possible causes of weakness are toxic disorders (botulism), exposure to an insecticide, or shellfish poisoning. Weakness that is related to an acute condition may require emergency attention. Seek immediate medical care (call 911) if you experience sudden onset of weakness on one side of your body or face; weakness with shortness of breath or palpitations; or weakness with loss of consciousness, severe chest pain, back pain, or abdominal pain. Seek prompt medical care if you have malaise along with other symptoms, such as abdominal pain or cramping, fever and chills, foul-smelling urine, or a general ill feeling.