I'm on the other side of this. The kid that was actually ill with weird or unrelated symptoms that seemed fine so no one really believed me including my parents. Or if they did they didn't think it was serious.
It started with side/lower back sharp pains that were intermittent and eventually went away. That was put down to attention seeking, hurting myself and not remembering etc.
Then I was complaining of being breathless, feeling faint, having to sit down just when going for a walk. This was put down to me being overweight /unfit and since I still wanted to go out and play must be fine.
Then I started looking kinda unwell so I saw a couple of paediatricians. They listened to my chest, claimed it was viral and it'll go away. I didn't have any cold symptoms besides still breathing kinda funny.
Then the high fevers started 40/41 and it was finally agreed that I should have a chest xray after much uhmmming and ahhhing whether it's really necessary and the risks of an unnecessary xray.
Once the xray came back it showed my left lung was 75% filled with fluid (pleurisy I think it's called?) and I was admitted right then and there. By then I was lethargic,stopped eating, refused to leave the bed except going to the toilet. I was on antibiotics, meds (with the next step being the tube draining thingy if no improvement) and a shit load of other meds. I was in hospital for a month, had to had physio to recover lung function, on meds for a very long time after and excused from PE for a whole year(that was a bonus tbh).
All because I was 13 and behaved like it (wanting to go out and be with friends and play)and looked well and was a bit chubby.