Probably cleaning too much. You need to be exposed to germs to become immune to them. I have never been a bleach cleaning, scrubbing, lysol spraying mom! (Unless someone comes home with a stomach bug!)
My kids are hardly ever sick. I just let the sickness run its course, wash sheets, clean toilets, and go on.
Also, I read it is not necessary to get new toothbrushes in some doctor article recently.
Also, I hear more and more doctors saying that hand sanitizers are HORRIBLE because more and more kids are not building up their lifetime immunities to the things they should.
Again- I'm NOT that germaphobe mom, and my kids are honestly like NEVER sick! We also do chiropractic treatments. In fact, the last time a tummy bug did hit our house, it lasted for literally like less than half a day. I think everyone only threw up once and had an ill feeling stomach for half a day. Meanwhile, my kids' classmates were missing 2 days of school or more. :)