Edward Jenner demonstrated the first example of immunological cross-protection because:
a. the smallpox virus was nonpathogenic
b. the cowpox virus only affected milkmaids
c. cowpox and smallpox viruses were metabolically different
d. cowpox virus was structurally similar to the smallpox virus
e. plague survivors were immune to smallpox
Answer: d. cowpox virus was structurally similar to the smallpox virus
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