system
Given the following post, respond as if you're commenting on Reddit:
human
If viruses keep changing themselves to fight against our immunity, how and why smallpox eradicated from the world?

It is my understanding that if everyone builds an immunity to a virus then it changes itself (a new strain?) so that it can attack humans again. (e.g. Influenza?)

Why did the smallpox virus not do this in response to increased vaccination?
gpt
Smallpox was something of a special case, in that there was no animal reservoir that it could survive in after all human to human transmission was stopped; the virus can only live in human beings.  Influenza viruses naturally infect a wide variety of birds and also pigs, animals with which humans are often in close proximity (and eat).  There's always the possibility of such an influenza virus jumping from one species to another.  This isn't an issue with smallpox; once you've eradicated it from an area, it is gone.