Oh, mama. I am still recovering from being terribly ill myself and I only have a 2 year old, I am a SAHM...and he's the one that keeps getting me sick!
Ok, so really all I can say is that this is kind of the nature of the beast. I hate to tell you. You're not alone and YES is really stinks that baby is sick. Nothing worse. There are lots of parents who go through this and if you're doing everything you can, then that's all you can do. Running away certainly isn't the option because baby would be getting sick anyways when she's in preschool and your other munchkins need you. Your baby is building her immunity along with her older siblings and that's just the burden of her birth order. It won't last forever. She'll be older and healthier in no time.
And I agree with the others sentiments. Don't overdo the sanitizing...obviously, clean door handles and light switch plates--frequently touched places...but don't overdo it with antibacterial stuff.
This is a good article, but give a look to google for more info on antibacterial soaps ill effects. The only place I have anti-bacterial soap is in the kitchen and I use it when preparing meats or working with eggs.
http://abcnews.go.com/Health/story?id=118140&page=1