Well she did have a ruptured spleen once, so there could be some compromise to her immune functions there.Disclaimer: I know literally ZERO about JN's medical.
I wonder if she has a 'compromised' immune system (genetically or other). The lifestyle of a pro racer means quite a bit of travel, and exposure (to germs and pollen).
Then again, most people get the flu nearly every year so maybe covid is the same for some?
I hope that she gets it figured out!