Another counterexample is the flu vaccine. It never purports to eradicate flu, but it helps with the season. The unfortunate reality is that some diseases chang

Another counterexample is the flu vaccine. It never purports to eradicate flu, but it helps with the season. The unfortunate reality is that some diseases change and evolve quickly enough that the immune system can’t defeat every new variant, even with bolstered protection against previous ones. Like you’ll catch a cold from time to time, or perhaps the flu. The immunity you developed previously didn’t stop that because the disease evolved.