I have not checked boxes for transient issues related to a known acute condition like an upset stomach due to flu. That's not really what they mean or want, since then everybody would check every box. They are really interested in more significant conditions than acute infections.

You would report a doctor visit of any sort, even if it's for something minor like flu, cold, etc.

But be careful deciding which boxes not to check... migraines and kidney stones are actually very significant for certification, as are ulcers, etc.

But if you had a cold/flu/sinus infection/montezuma's revenge, etc and it resolved itself without a medical visit then you don't need to do anything with that. It won't affect anything and they'll never know (or want to know). Of course you ground yourself while sick.