Infant care sick madness

My daughter has been in Infant care ever since she was 5 months. Ever since then, every month she is down with the usual cough, flu, fever. And really took super long to recover. In the end I feel like i’m wasting my bills there. Kept giving her vitamin c but doesn’t rly works. I can’t take her out because both my husband and I are working and we had no choice. Just wondering how long doe sit takes for children like this to have stronger immunity? Breaks my heart everytime she’s like this