Question

how did Edward jenner discover vaccination?

Answer 1

he saw that the milkmaid were resistant to a disease called small pox because they had a similar type of weak bacteria present in them called cowpox

Answer 2

once he saw some pox like structure on a cow and discovered that it is a small pox which is also called cow pox and from their e got an idea for the discovery of vaccination. this was possible bcus of the saying goes curiocity is the mother of invention.
i hope it will help you !!!!!