When I was in primary school, my perfectly healthy friend lost his eyesight almost completely over a week or so. He does see vague shapes but is legally blind. Back then, the adults told me that it was due to flu complications. Is this actually possible? If so, what would be the mechanism?

This is gonna be very anectodical and I can talk mostly for Poland - Europe can differ vastly from country to country. From my perspective, I don't see as many rude or entitled customers as it seems there are in the USA. But on the other hand, from a North American perspective, customer service is rather cold here at best and rude at worst. There are exceptions of course.

You may like it or not but it's been a thing in Poland to an extent for decades, even in the interwar period when we weren't nearly as influenced by American culture. So it's by no means a recent cultural import from the USA.

Data source: this Reddit poll that I made some time ago: https://www.reddit.com/r/poland/comments/x297te/do_you_tip_in_restaurants_in_poland/ .
Tool: Google Sheets.

The results must be treated with a grain of salt: I got 2393 answers and the subreddit's community is most likely far from being a representative slice of Polish society. The poll was also quite simple due to Reddit polling limitations.

Nevertheless, the results are interesting. The respondents were divided almast perfectly into thirds both on the amount they tip (10%+, just a few PLN, nothing) as well as the frequency (always, only when the service was exceptional, never).

Bear in mind that there's no tipping culture in Poland akin to USA. Tipping is optional and happens mostly in restaurants with service if ever. Looks like it's a hot and divisive topic anyway.