Realistically there's nothing to prevent it. Clean more, open windows for a bit if you can, make sure the 3yo washes their hands (a constant battle over here). I have my 3yo shower when he gets home from school, I clean his school bag and I wash his clothes as soon as I can. We got an air purifier it seems to help. I also started doing a multi vitamin, elderberry and probiotic for him.
Let's Glow!
Achieve your health goals from period to parenting.