Maybe the question isn't why some viruses exist, but why for some people they're no problem. What's the pertinent difference?

In every epidemic not 100% die. So why? What do those who experience mild or nothing have which others lack? In essence what are we missing?

When as a child you encounter a kid who as a consequence of being given the polio vaccine, gets the full blown polio, not to mention the cases in India in the past few years directly related to the polio job, (renamed to hide the fact there was a direct relationship) its obvious something is going on. But what? That's is a different matter.

Not unlike, comparing a dot or full stop, a circle, a doughnut. Depending on your size say an ant, their world is huge in relation. Now for us, the dot, circle, doughnut, no biggie. But just as they can't experience what we perceive, so too it could be why we can't discern why gravity is so weak on our planet. Plus just as the ant is confined so are we, due to our size. For all we know we too are like the ant, unaware of something towering over us.

Basically its about how things are interpreted. Have you ever tried to translate from one language to another? Some words in other languages just don't exist! Literally. For example a woman was shocked at seeing someone nibbling food left by another. Asked why? She said it wasn't the done thing. Left overs wasn't the correct translation. Because in her language the word used implied you didn't consume someone else's left over food, drink etc. Also a direct translation, even if clumsy, or a more lyrical one to convey the feelings behind the phrase? There're choices.

The current definition of virus. https://www.medicalnewstoday.com/articles/158179