Final answer:
Cowpox provides cross-immunity for smallpox, and this relationship was vital to the development of vaccinations that led to the eradication of smallpox in 1977.
Step-by-step explanation:
Cowpox provides cross-immunity for smallpox. This concept became a foundation for the development of vaccines. In particular, Edward Jenner's work in the late 18th century demonstrated that individuals who had been infected with the cowpox virus were subsequently immune to smallpox. Cowpox and smallpox viruses are closely related, enough so that the immune response to cowpox can provide protection against smallpox. This cross-immunity was critical in the eventual eradication of smallpox from the human population in 1977.