He found that they'd all contracted a similar but milder disease, cowpox, which vaccinated them against the deadly smallpox.
他发现他们都得过一种相似的但是温和的小病，牛痘，正是由于牛痘使得他们逃过了致死的天花。
2
An English physician, Edward Jenner, discovered that by inoculating a person with pustules from the pox infection of a cow, smallpox could be prevented.