I was working full time when my 12 year old was a baby, up until he was 2 1/2 years old. He was at a Day Care. I felt like I was taking him in to the pediatricians office alot. He would get sinus infections, ear infections (from the sinus infections), coughs, stomach flu, I felt like it never ended. IF you live near the Seattle area, we are experiancing the coldest spring we have had since 1917. So cold and flu season I think are extended because of our unusual cold weather. When summer came, I would get a little break from the cold season. When it starts to warm up, we (and our kids) don't get as sick. However, having your kids at the sitters, with a bunch of other kids, who have colds-your daughter will get sick. When I quit my job, and my son wasn't going to day care any more, he rarely got sick. Day Care kids I feel don't have a chance of not being sick. Parents have to go to work, and unless thier kids are running a fever or vomiting, they take thier kids to Day Care exposing all the other kids. It's a vicous cycle, that I always felt, I was never going to get out of.
You could try giving her Childrens vitamins and multi vitamins (from a GNC or health food store), don't give her Flintstones-they don't work.
Good Luck.