The term underweight refers to humans that have much less weight than they should. Being underweight is bad for the overall health of the person affected. It is normal what a person's weights changes over time. Being underweight is often defined using a measure called the body mass index which calculates a score, based on height and weight. If this score is below 18.5, a person is generally said to be underweight. This is only a general indication, though as the scores must not be calculated for everyone. It does not apply to children and adolescents, for example.