Nobody actually knows how many people died of Spanish flu. Look at the range of estimates provided and where they come from - you'll discover that the distribution is enormous, partly because the high estimates come from epidemic models. But such models always seem to overshoot by large amounts, so their estimates aren't credible.

Putting that to one side, Spanish flu happened in the context of:

- A society that didn't know viruses existed at all, only bacteria.

- The very recent discovery of aspirin, which was believed to be a wonder drug that could cure anything. Doctors didn't know what doses were safe and had a habit of prescribing lethal overdoses to people suffering from the flu.

- Widespread censorship by the authorities due to WW1. It became known as "Spanish Flu" mostly because the censorship was just less aggressive there, so that's where reports emerged from first.

For tech firm employees systematically censoring reports of bad reactions to vaccines, there are lessons here that still apply to the modern day. Spanish Flu would probably have been less deadly in a society with less censorship, and which was less likely to mass prescribe brand new miracle pharma products.

I do think this is an appropriate counter-example, but I’m curious how much modern medicine would change the impact (and preserve the case for optimism).

Rapid vaccine development does indeed give some cause for optimism. Some counters that I'd have to that would be a potential resurgence of polio in the US [1] and the appearance of drug-resistant pathogens (typhoid fever in this case) [2], just to keep us on our toes and curb our enthusiasm...

[1] https://www.cdc.gov/mmwr/volumes/71/wr/mm7133e2.htm [2] https://wwwnc.cdc.gov/travel/notices/watch/xdr-typhoid-fever...

The Spanish flu also happened in a much less medically advanced society, that didn't understand how viruses work

Bacteriology was already developing at that time, first vaccines were already in use (for none other than a viral disease!), and the existence of a pathogen smaller than bacteria was already hypothesized. The response to Spanish flu was also eerily similar to what was recommended in the first year of COVID: https://en.m.wikipedia.org/wiki/Spanish_flu#/media/File%3AIl....

All that aside, your comment does not really prove that "nature cannot come up with something so transmissive", which was the claim here.