Many diseases can cause extreme fatigue and weakness. According to MedlinePlus, a service of the U.S. National Library of Medicine and the National Institutes of Health, fatigue is a feeling of weariness, tiredness or a lack of energy, and fatigue is different from drowsiness. MedlinePlus states that drowsiness is the feeling associated with sleep deprivation, whereas fatigue is the lack of energy and motivation. Weakness often accompanies fatigue, and both can affect a person's quality of life.
Anemia can cause extreme fatigue and weakness. The Mayo Clinic website reports that anemia occurs when a person lacks sufficient red blood cells or iron to carry oxygen to her tissues and organs and that a person with anemia will experience weakness and fatigue. Anemia can be caused by many factors, and there are numerous types of anemia. The Mayo Clinic website states that, along with fatigue and weakness, common symptoms associated with anemia include the following: pale skin, a fast or irregular heartbeat, dyspnea or shortness of breath, chest pain, dizziness, cognitive problems, cold extremities, and headache. Anemia can be caused by iron deficiency, vitamin deficiency, chronic disease, bone marrow disease, autoimmune disorders and sickle cell anemia, among other causes. Anemia can be temporary or chronic, and its severity ranges from mild to marked.
Cancer can cause extreme fatigue and weakness. The Cleveland Clinic---one of the top four hospitals in the United States---states that cancer-related fatigue is one of the most frequent side effects of cancer and its treatment and that it's not predictable based on tumor type, treatment or phase of illness. Cancer-related fatigue typically arises suddenly, is not associated with physical activity or exertion, and is not made better by rest or sleep. According to the Cleveland Clinic, people who experience cancer-related fatigue describe it as paralyzing, and it often continues after the cancer treatment has ended. Along with chemotherapy, radiation therapy and other cancer treatments, the disease process itself can cause extreme fatigue and weakness. The Cleveland Clinic states that tumor cells compete with normal cells for nutrients, usually at the expense of normal cell's growth and development. Weight loss and a decreased appetite---both common symptoms of cancer---also cause fatigue and weakness.
Malnutrition can cause extreme fatigue and weakness. According to MedlinePlus, malnutrition occurs when a person's body does not get enough nutrients, and malnutrition may be caused by an inadequate or unbalanced diet, digestive or absorption problems, and certain medical conditions. MedlinePlus states that, although malnutrition-related symptoms vary based on the type and severity of the condition, common general symptoms of malnutrition include the following: fatigue, dizziness and weight loss. The Mayo Clinic website states that muscle weakness is especially common in malnourished older individuals and that muscle weakness can lead to falls and fractures in this population. According to MedlinePlus, malnutrition is treated by replacing missing nutrients and addressing any underlying conditions, and the prognosis or the expected outcome of treatment depends on the cause of the malnutrition.