It is different from fatigue , which is a loss of energy. Weakness can be used to describe a mental and physical state in which someone doesn't have the muscle strength, for example, to walk. It is common and sometimes difficult to evaluate.
Weakness has many causes.
Weakness itself may be a symptom. Some examples are multiple sclerosis , which causes inflammation and damage to the brain and myasthenia gravis , which causes muscle weakness that often gets worse toward the end of the day hormone imbalances, such as low thyroid hormone levels, called hypothyroidism , or low adrenal hormone levels, called hypoadrenalism any infection, especially infectious mononucleosis, flu , poliomyelitis , botulism or pneumonia any serious diseases, such as congestive heart failure , cirrhosis , chronic renal failure , or cancer psychiatric conditions, especially depression chronic fatigue syndrome and fibromyalgia , two poorly understood conditions with no known cause that commonly make people feel weak and tired Other causes are also possible. Sometimes, no cause is found. Prevention depends on the cause. For example, weakness due to lack of exercise can be prevented with regular exercise.