Hey ladies, has anyone gone through this and can share their stories and experience? Just to clarify…

Hey ladies, has anyone gone through this and can share their stories and experience? Just to clarify, this isn't about my younger one. It's been a month since school started, and my older daughter is already sick for the third time. She's turning 9 in a couple of weeks. First, I caught something mild, you know, just a cough and a slight fever (100°F). I got better, but then my younger one brought home a virus from daycare, with a 103°F fever and a fiery red throat. Her whole class got sick with the same symptoms, and my older daughter caught it too. She spent 2 weeks at school, and yesterday evening and through the night, her fever was back up to 103°F. The doctor came, said her lungs were clear, throat too, just a few coughs and a little stuffy nose. As a very anxious mom, I've already spiraled completely. Our pediatrician is coming back from vacation soon, and I'll ask her for a list of tests to run to figure out why her immunity is so low. I'd also like to check her digestive system, there are some things that worry me. She has no appetite, her diet is terribly poor, so how can she have immunity if her nutrition is so bad? But maybe some of you have kids who get sick often or have been through this? Maybe you can share some ideas on how to improve the situation? Thanks, everyone!