Wasting is a weight too low for the length and a measure of malnutrition that has arisen in a short period of time.
Usually this is a result of inadequate nutrition combined with a high incidence of infectious diseases, especially diarrhoea. Conversely, wasting leads to a reduced functioning of the immune system resulting in a more severe and prolonged course or risk of developing infectious diseases resulting in an increased risk of death.
Underweight, on the other hand, is too low weight for age.
The more serious the underweight, the greater the chance of mortality.