Exposure to germs

Hoping you all can help me settle a debate I'm having with my husband (more like an argument, actually). At what age should you start exposing your baby to germs to "make them strong"? My son is 9 weeks old and my husband is acting like I'm a paranoid nag for asking that he wash his hands after he gets home before he touches the baby. He's a teacher, if that matters. I also don't want our dog (who eats her own poop) licking the baby. He keeps insisting that our son needs the exposure. I insist that he's still too young and that it's flu season. So...am I being unreasonable?