

I’m not sure I completely understand the question. It sounds a bit like “How do you enjoy a nice meal” - there’s no way to instruct someone how to appreciate or enjoy something, you just have to take a moment.
The thing with the “before it’s gone” part is that we don’t always know which things are temporary, so you just have to cultivate an attitude of appreciating things in your life, including the people in it.
COVID/flu symptoms are the same symptoms as a lot of things, so hard to say anything about what they have based on those symptoms. If their doctor is decent, your best advice is to just let them do their thing. You’re not going to get a useful diagnosis from the Lemmy community.
I should stop there, but I came here to answer the title of your post. Yes, my dad died of something they never figured out. Was very strange.