Maybe this is a well-documented phenomenon, but it surprised me...
Over the weekend a family member was feeling under the weather and had a temperature. I looked in on him sleeping and thought "it smells like a sickroom in here" even though the bed had been stripped and changed that morning, he'd had a shower a few hours ago, and he hadn't been sick or anything. It just had THAT smell. Fast forward a few hours and he was at the hospital with a serious stomach bug (he's OK now, btw).
I'd heard of dogs being able to smell illness, but not people so much. I thought the 'sick room' smell was a combo of sweat/vomit/cleaning stuff, but I googled and it turns out that various elements of your immune system going into overdrive can cause a distinctive smell. It's something I'll definitely pay attention to in future!
Just wondered if anyone else had any experience of this phenomenon?