First let me address your completely incorrect assertion - "It is established science that if you get infected by a virus, be it smallpox, measles, etc. you become immune to it once, you are cured of it."
The initial concept of herd immunity came out of medical studies of the measles epidemic in Baltimore in the 1930s -
papers from Sencer et al and Hedrich and other found the following:
- In one of the most deadly outbreaks of measles in the U.S. in the 1930s in Baltimore; it appeared that 53% of the population acquired natural immunity after recovering.
- This 53% immunity of the population was enough to slow the spread of the virus but not eliminate it.
- There were individuals who recovered from measles who caught it a second time, but the percentage ws small.
- The theory of using vaccines to create "herd immunity" against a disease came out of these observations.
- However "herd immunity" requires that 70% or more of a population to be immune to stop the spread of disease.
- Naturally it is difficult to get much beyond 50% heard immunity in a population. A vaccine is needed to boost the level to 70% or more.
- Part of the theory of "herd immunity" is that you need to protect the individuals who did not acquire immunity from vaccines (or exposure), are too young for the shots, or did not get vaccinated. There will always be a group in society in this position.
Now let's directly address your absurd claim - ""It is established science that if you get infected by a virus, be it smallpox, measles, etc. you become immune to it once, you are cured of it."
- Only a percentage of people who catch a disease are immune to catching it a second time.
- Admittedly this percentage for measles is pretty high for who are immune.
- But one of the reasons it is difficult to get much beyond 50% herd immunity naturally is because of the small percentage of people who do not become immune after recovering.
- Even with vaccines there is a percentage of people who get the vaccination who don't develop the immunity to the disease. Which is why herd immunity is important to protect them.
- There is also a question of how long immunity lasts after recovering. For some diseases it is only a matter of months, for others years. The reason people need vaccine boosters is because even vaccinated immunity wears off over time.
Let me use a personal example. I received the MMR vaccine (measles, mumps, and rubella) as a child. I did not show immunity to mumps after the shots. So the gave me more doses to see if immunity to mumps would develop. It never did - even with further shots as an adult I am still not immune to mumps. I am one of the 3% or so of American society or so which is counting on herd immunity to shelter me from mumps,
As follow-up reading for COVID-19...
Coronavirus immunity may only last six months raising doubts over 'immunity passports'
The discovery by scientists at the University of Amsterdam has raised questions over whether 'immunity passports' could be a route out of lockdown
https://www.mirror.co.uk/news/uk-news/coronavirus-immunity-only-last-six-22074166