Why do some children fall ill more frequently

Why do some children fall ill more frequently than others living in the same locality?

Children fall ill more frequently due to
(i) poor personal hygiene;
(ii) poor domestic hygiene;
(iii) unclean food;
(iv) lack of proper nutrition and balanced diet.
All these factors result i(i the weakening of the immune system, which makes these children more susceptible to disease.