Why is she always sick!?

I always ask myself how can she always be sick? She was NEVER not even with a cold sick before starting school at 3.5 years old. Now she's sick all the time when she is in school but during breaks she is fine and healthy then when back to school bam sick again. I know it's good for her now to build up a immune but it's getting tiring how sick she gets. Is it mold in the school, is it something in our house ( it couldn't be because during school breaks we are always home so she would be sick during breaks as well but she isn't) I don't know if it's just allergies i just don't know what it could be.