You are wrong. 2/3 of the world’s population did NOT get infected with Spanish Flu, survive, and gain immunity.
Do a little research before spouting off.
The 1918-19 Spanish Influenza Pandemic and Vaccine Development
September 26, 2018
https://www.historyofvaccines.org/content/blog/vaccine-development-spanish-flu
“First, the numbers. In 1918 the US population was 103.2 million. During the three waves of the Spanish Influenza pandemic between spring 1918 and spring 1919, about 200 of every 1000 people contracted influenza (about 20.6 million). Between 0.8% (164,800) and 3.1% (638,000) of those infected died from influenza or pneumonia secondary to it.”
So, with no vaccine, and no herd immunity, Spanish Flu faded out.
First, you are counting only those afflicted with the virus in the US. The figure that I gave you was estimated broadly, because it was impossible to know how many died of Spanish Flu, let alone contracted it. So 17,000,000 to 50,000,000 out of 500,000,000 who contracted it is the best anyone can do. Can you say “mass graves,” class?
Second, you do not need 2/3 of the population to get infected to get herd immunity. Spanish Flu was not as virulent as the Black Death, and it’s debatable how big a chunk of the world population had to get infected with the Black Death in order to bring about herd immunity. In the 13th Century, hygiene was unspeakably bad, nobody knew that there was such a thing as viruses, and attributed contracting diseases to “miasmic humors”. They had no health care system to speak of, partly because medical practitioners ran the risk, if their patients died, of being lynched as a sorcerer, and those medical practitioners did not know much about medicine in the first place.
Let’s say that 2/3s of the world’s population contracted the Black Death and between 1/3 and 1/2 of the world population died of it. That does not prove one way or the other how many people have to contract the Spanish Flu in order to establish herd immunity, or even how many have to contract the Black Death to gain herd immunity. You have to also know how often a virus mutated, and how efficiently it was spread. Do you know how many rats infested London as opposed to Paris in the 13th Century? Or how many Crusaders returned to either city after the fall of the Christian colonies in the Middle East?
We do know that mitigation helped. Jews, who had relatively better hygiene than poor Christians at that time, contracted it less often, so were blamed as its instigators, and instead of dying of the plague died of the pogroms that ensued from not getting the plague. But from that tidbit we do know that keeping your home relatively safe from rats and mice will tend to make it harder to get the plague, just like not peeing in the gutter outside your door will help reduce your chances of getting typhus.