I was watching a television show yesterday and the premise of the episode was that a terrorist group had broken into an old abandoned USPHS lab and stole samples of the original strain to use as a biological weapon. It got me thinking, is that particular version of the flu virus still particularly dangerous? I know H1N1 strains are still dangerous and have been responsible for a few more pandemics since the Spanish flu but it seems that we should have some resistance to the strain that caused that pandemic. My reasoning is that it never went away. We didn't beat the Spanish flu with vaccines and health measures rather it just killed pretty much everyone it could and we eventually developed a level of resistance to it that made its threat more in line with the seasonal flu. If my reasoning is correct then the terrorists releasing the virus in the subway shouldn't be any more dangerous that someone with the flu taking the subway to work which is a common occurrence during flu season.
So, how does it actually work? Did we develop a resistance like I think or would a release of the original strain start a new pandemic?
The original Spanish Flu would absolutely fuck us up.
The reason it became mild has nothing to do with us building immunity but because a virus killing its host is bad for business (or strictly speaking, reproduction). A lack of viable hosts puts pressure on the virus's gene pool and in the end, the variant that is most successful at spreading and reproducing will win out.
That means not killing your host and only doing mild, repairable damage to other potential hosts so that humans don't take an infection so seriously.
We saw this exact pattern with COVID, with successive iterations being less deadly than the last.