"The term underweight refers to a human who is considered to be under a healthy weight. The definition is usually made with reference to the body mass index (BMI). A BMI of under 18.5 is usually referred to as underweight. It is important to note that the BMI is a statistical estimate and some individuals classified as underweight may be perfectly healthy. In fact, caloric restriction may be a viable means of increasing the lifespan, and it can easily lead to a BMI of less than 18.5. This medical definition of underweight may differ from other uses of the term, such as those based on attractiveness.
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Severely underweight individuals may have poor physical stamina and a weak immune system, leaving them open to infection. According to Robert E. Black of the Johns Hopkins School of Public Health, "Underweight status ... and micronutrient deficiencies also cause decreases in immune and non-immune host defenses, and should be classified as underlying causes of death if followed by infectious diseases that are the terminal associated causes." People who are malnutrative underweight raise special concerns, as not only gross caloric intake may be inadequate, but also intake and absorption of other vital nutrients, especially essential amino acids and micronutrients such as vitamins and minerals. In women, being grossly underweight can result in amenorrhea (absence of menstruation) and possible complications during pregnancy. It can also cause anemia and hair loss. Underweight is an established risk factor for osteoporosis even for young people. This is a particular insidious consequence, because the affected persons do not notice the danger, they can feel fit and may be brilliant for example in endurance sports. After the occurrence of first spontaneous fractures the damage is often already irreversible." (Wikipedia)