Tathagata Haldar (anonymous) doubts approved

sir, I have one doubt, I have posted on a question before the name of the question is how vaccination works and there you have written that as far as cowpox vaccination is concerned, it might have happened that this vaccination could not have killed all the bacterias as result people would have suffered from little cowpox.Sir, you please check that it is cowpox bacteria or cowpox virus.Sir, I am very confused.Please explain.