Why are we normally advised to take bland and nourishing food when we are sick?

$A$ doctor,nurse,or health worker is exposed to more sick people than others in the community. Find out how they avoid getting sick themselves.

How many times did you fall ill in the last one year? What were the illnesses?
$(a)$ Think of one change you could make in your habits in order to avoid any of/most of the above illnesses.
$(b)$ Think of one change you would wish for in your surroundings in order to avoid any of/most of the above illnesses.

Under which of the following conditions are you most likely to fall sick?
$(a)$ When you are taking examinations.
$(b)$ When you have travelled by bus and train for two days.
$(c)$ When your friend is suffering from measles.
Why?

$A$ baby is not able to tell her/his caretakers that she/he is sick. What would help us to find out:
$(a)$ that the baby is sick?
$(b)$ what is the sickness?

Are having a balanced diet and maintaining personal and public hygiene the same or different? Why?