1. Vaccination is efficacious in preventing smallpox.
    种痘对预防天花很有效。

2. Smallpox was once the scourge of the world.
    天花曾是世界的大患。

3. Smallpox was once the scourge of the world.
    天花曾是世界的大患。

4. I´m immune to smallpox as a result of vaccination.
    我种过牛痘了, 所以对天花有免疫力.