Why my eldest daughter takes longer to shake off simple illnesses?

[](https://www.reddit.com/r/askscience/?f=flair_name%3A%22Human%20Body%22)Our first daughter is almost 6 years old, meaning she was born on the height of COVID-19. Ever since she was born, especially when we were still partially in isolation mode, I've noticed that she will take almost twice longer than me or my wife to bounce off after catching a cold, or whatever. She will come home from kindergarten (because of 'covid childhood' we opted to start kindergarten early, before she was even 2) with something, one of us will catch it from her. Days after she will still have symptoms while we are already back on our feet. I'm writing now, free of some kind of a sinus infection she gave me, while she still coughs and her nose is still running. I wasn't even treating myself so well, I was just stealing from her prescribed medicine whenever it got too serious. We always thought it was her age, but since having a second daughter, 1yo, we've noticed that the baby actually shakes off a cold quicker than us, meaning possibly 3 times quicker than our older child. Both are following vaccination routine. What is going on and how can I help my older daughter?