Honestly, I think it's a mix of both lah, some kids memang ada yang strong constitution dari kecil, tapi routines and environment also play a big part, so if your child is thriving at the factory/industrial area without getting sick, it could be good genetics, good habits (hand washing, sleep, nutrition), or just lucky timing yang dia avoid the common bugs going around! A few of my mum friends noticed their kids punya better immunity bila they're in more varied environments compared to staying home je, so it might not be just "rezeki", could be that exposure itself is building up their system lah.