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

(N/A) Some children have a weaker immune system due to various factors.
$1$. $A$ child may not be receiving a balanced diet,leading to malnutrition.
$2$. $A$ child might be living in a congested and unhygienic environment,increasing exposure to pathogens.
$3$. $A$ child may not have access to clean drinking water,which increases the risk of water-borne diseases.
These factors make certain children more susceptible to falling ill compared to others living in the same area.