How Dos A Kid Get Sick In One Day?

I believe it's what their exposed to, meaning, Kids put their hands in their mouths without washing their hands, they visit relatives, the might be around things that has lots of germs like water fountains, door knobs, crayons, pencils, or anything can be passed from 1 child to another.  Sometimes it can be things in the air.  When I was in high school, my biology teacher told me something I would never forget, She had the windows open in the middle of december and kids were complaining about it being cold and that they will caught a cold.  Her response was" it's not the cold that gives you a cold, it's what's in the air".  That made a lot of sense to me.