Hey there J.. Here's one more consideration for you, how much time is your child spending outside in this cold weather? Years ago, when I started teaching preschool, I supplemented my income by subbing at daycare "preschools". What I observed is that some of these centers FORCE the children to go outside every day -- as soon as they hit the 2s. Freezing mornings, as long as it was dry, the kids were essentially locked outside to "go play". Often times coats were not even put on them. I was shocked to say the least. I think we can all figure out how that impacts a sick little child's immune system.
The reason for this practice was that the under-paid, under-trained, and over-tired teacher's aides (the bulk of their workforce) wanted to wear the kids out so that they would come in, eat lunch and then settle down for a lonnnnnng nap -- sparing them from caring for the children for the bulk of the afternoon.
I would recommend talking to the director about your situation. Question their sanitation practices, as another mom suggested. How rigorous are they about having the children wash their hands before meals for example? (This alone would do it.) Do staff wear gloves and wash hands after helping each child with toileting/diapering? How are tables cleaned before and after meals? (Both a detergent type cleaner and a spray disinfectant should be used.)
But also ask about the daily routine. How much time does your child spend outside on a winter morning? Ask if you can come and observe. And see what kind of response you get.
It may be time to find another daycare arrangement.
Although it is normal for them to go through a phase of being sick more often (because of exposure to so many new germs), this sounds excessive. Especially as it is always the flu, not varied with lesser colds, etc.