Sick Sick Sick of Winter!!!

How in the world do you keep your kids healthy??!!
This has been a tough winter for us health-wise. It seems like one of us is sick at any given point. It's McKenna's turn right now - the poor girl cannot breath through her nose, doesn't want to eat, and looks like her eyelids weigh about 10 lbs each. And actually, Dave came home from work last night not looking so good. He says he feels like he's getting the flu. Oh great. I swear, I clean our toys regularly; clean all the door knobs, phones, faucet handles, and keyboards with antibacterial wipes; we wash our hands frequently; I'm giving the girls lots of Vitamin C, fruit, and juice; we don't go anywhere; we haven't had very many playdates; and STILL, we keep getting sick! What else can I do? I am sick of us being sick! The past few days have been really nice outside and I've tried to keep the windows/doors open as much as possible to get fresh air in here - hopefully with the Spring will come healthier days for the Jensen Family. Boy, do we need it!