I've had an old habit of veering towards "stomach flu", as you're not expected to have symptoms after you get better. Also IF your boss randomly asks for a doctor's note, the doctor isn't going to stick a finger in your butt to make sure that you, in fact, have diarrhea. And the doctor is likely just going to give you the note.
My problem is that :im-vegan:, so when I'm constantly calling in for stomach bugs, my workmates are always saying "I thought you guys don't get stomach parasites". Also I'm literally the most covid paranoid person I know IRL, N95 mask everywhere and eating in my classroom alone, with windows open if possible. So it's a lot less likely that I'm going to get a cold or the flu.
So what am I going to do next time I need a lie-in? Say I broke a rib? UTI? Aggressive acne?
My ranking and never be descriptive:
Gastro is best because there's no easy proof, we've all been there, and it's embarrassing. Vomiting, diarrhea, etc
Head symptoms are good because, while you might get some follow up questions and advice, there's no easy proof and we've all been there. Migraine, dizzy, etc.
Debilitating symptoms can't be easily proven, but they get way too many follow up questions and people might push back if they can't imagine what you're describing. Leg cramps, thrown back, etc.
:order-of-lenin:
Good high effort answer