If someone is undernourished, they are weak and unhealthy because they have not been eating enough food or the right kind of food. undernourished children. = underfed, malnourished. unhealthy and weak because you have not had enough food or the right type of food = malnourished
If someone is suffering from undernourishment, they have poor health because they are not eating enough food or are eating the wrong kind of food. = malnutrition