How to develop immunity for kids?

My niece is 4 years old. She is slightly lean for her age. She used to catch cold once in a month. But lately she is getting sick almost 3 times a month. Feeding her is a problem but somehow she manages to eat a meager amount of food. How to develop her immunity? what kind of food should we give her to avoid cold? answers from doctors or experts or experienced parents are most welcome. please provide sources too.