- sunnyLv 51 decade agoFavorite Answer
Underweight individuals generally 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 defences, and should be classified as underlying causes of death if followed by infectious diseases that are the terminal associated causes." They are also at higher risk from heart disease, blood vessel disease, and osteoporosis.
In women, being underweight can result in amenorrhoea (absence of periods) and possible complications during pregnancy.