Smallpox is a virus with a fatality rate of about 30%, and those that survive the disease have terrible scarring and lingering problems. Edward Jenner is famous for a now notorious experiment in which he developed the first vaccine for smallpox. In the experiment Jenner first exposed a small boy to cowpox, which is relatively harmless. The boy got cowpox and recovered. Jenner then exposed the boy to smallpox by walking him through a hospital ward. The boy lived. Which conclusion is NOT valid? A) Cowpox virus can be used as a vaccine against smallpox. B) Immunity to cowpox virus also grants immunity to smallpox. C) Cowpox virus and smallpox virus are probably very similar. D) Smallpox virus would be a good choice for a vaccine against cowpox.

The correct answer is "letter d. Smallpox virus would be a good choice for a vaccine against cowpox."

Based on the given statement, smallpox virus was never used as a vaccine and proved to be effective against cowpox, cowpox is known to be a harmless illness, which is why the boy was able to survive it. On the other hand, cowpox virus would be a good choice of vaccine against smallpox.

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