What observation made by Jenner contributed to the eradication of smallpox?

The observation that milkmaids who had caught cowpox did not get smallpox was pivotal in the development of the smallpox vaccine. Edward Jenner noted that cowpox, a disease less severe than smallpox, seemed to provide immunity against the more dangerous disease. This led him to hypothesize that exposure to cowpox could protect individuals from smallpox infection. By inoculating individuals with material taken from cowpox lesions, Jenner effectively demonstrated that this practice could confer immunity against smallpox, laying the foundation for the development of vaccination as a public health intervention. Thus, this observation was crucial in understanding the relationship between these two diseases and in promoting efforts to eradicate smallpox.

Other options do not directly contribute to the discovery of vaccination. For instance, while it is true that infected individuals may gain immunity after recovery, this does not relate to Jenner's innovative approach with cowpox. The idea that vaccines could be derived from plant sources does not apply to the context of smallpox, which is caused by a virus. Additionally, stating that smallpox was not contagious is incorrect, as smallpox is highly contagious, contributing to its widespread impact before eradication efforts were successfully implemented.