What part of the Chain of Infection was broken by the development of the Smallpox vaccine?

Without a chance to enter a new host, the disease is stopped cold. Breaking Link # 6: Resistant Host When the English doctor Edward Jenner developed a vaccine to prevent smallpox in 1796.

blood alcohol concentration is affected by factors such as body size and gender.

( i saw this one before)

the answer would be a." don't hit a home run when the game is tied"