Marasmus is a disorder of malnutrition that is a result of total calorie starvation, especially protein and energy. It is often seen in children under 5 years of age, most commonly in developing countries (although older children and adults can have marasmus). The decreased nutrition is usually secondary to weaning problems resulting from disease, poor hygiene, poverty, and cultural factors. Young children with repeated bouts of gastroenteritis are particularly at risk.

Patients are emaciated with loss of the subcutaneous fat and muscle. The skin is dry and loose. They also have thin, fragile hair and nail abnormalities.