This passage begins with an introduction to smallpox. It goes on to describe the work of Edward Jenner, in 1796, in developing a smallpox vaccine. The passage relates that Jenner noticed people infected with cowpox, a harmless disease, became immune to smallpox. Jenner conducted a test to see if this immunity could intentionally be granted to someone. He exposed a young boy to cowpox, resulting in a "slight infection." The keyword "Later" implies that whatever goes in the blank will build upon the idea of this test, which was to see if intentionally giving someone cowpox would make them immune to smallpox. Choice (A), the correct answer, does so, explaining that the boy proved to be immune to smallpox thanks to the inoculation with cowpox.
None of the other answer choices continues the discussion of that initial test, instead skipping ahead to after its completion. Incorrect choice (B) discusses the skeptical reception to Edward Jenner's vaccine concept. Incorrect choice (C) skips ahead almost two centuries, from 1796 to 1966, to discuss the global eradication of smallpox. Incorrect choice (D) also skips ahead to the widespread adoption of the vaccine model.
