Being underweight can affect your body systems in many ways. Because it affects your immune system just like this first person said earlier and it could cause some sort of disease that could take over your body. Being underweight always have its bad outcomes. For instance, if that person is in the situation where his or her weight is below average, then it could cause their strength to become weak and their diet could be in danger. It could also cause your body functions process slowed or stop its process.