Winter time is certainly very difficult. Lots of colds and flu going around, my pediatrician told me they've been seeing whooping cough in the elementary schools this year as well. Your older kids are exposed to lots and lots of germs...then they bring it home. You're probably run down from caring for a newborn... so that gets you sick and your little one... well, he's still developing his immune system so he's very vunerable.
Are you breastfeeding? Quite honestly, breastfeeding is one of the best defenses for little babies against getting sick. When my daughter was young, we lived in a big city. Although I don't have any other children right now, we rode the underground and were in contact with all sorts just about every day. She was never sick.
Also, have you considered your using too many antibacterial items? Our bodies need to be exposed to germs to build natural defenses. Germs get into our system and they don't always cause us to get sick, but often times our bodies just react to them internally and build a defense. That saying our grandma's used to have.... "a little dirt never hurt" ... well, it's actually pretty true.
Another thing, ever heard of Sambucol? It's an elderberry syrup and has studies behind it that strongly suggest it does help boost the immune system. I give it to my daughter every days. She's 19 months old, and is in contact with other kids. She's gotten sick a couple of times this winter...but overall ... she's been pretty healthy. Certainly nothing out of the norm, or to the extent I was really worried.
Make sure your kids are getting a good balanced diet full of fresh fruits and veggies. A good diet goes a LONG way in keeping us healthy.
www.nourishmd.com is an EXCELLENT source for info on nutrition. The site is run by a dietician and pediatrician. They are wonderful.
HOpe everyone is feeing better soon.