Government website on the flu: PandemicFlu dot gov. Also, from the CDC , WHO, Wikipedia and beyond, learn about Endemics, and pandemics and Epidemics.
Those of you who have read some histories of the Flu of 1918 (as opposed to reading just headlines), or some of the less overwrought news (e.g. at the CDC) may recall that there was a 1918 Springtime outbreak of flu that preceded the more virulent Autumn one. (I liked John Barry’s book, the Great Influenza, but there are others, many others.) You may also recall that those who caught the early, less virulent form either didn’t get the deadly one or had a better outcome if they did (they lived) than those who had not been exposed previously. They were, in purely layperson’s terms, “vaccinated.”
As my father used to say about Calculus 101. If you survived it, you had been vaccinated and were immune; you didn’t have to get it (take it) again. You might go on to get Calculus 201, but you had a better chance of survival than if you hadn’t been exposed previously to Calculus 101.