In the eighteenth century, it was observed that milkmaids were less likely to contract smallpox than others. What caused this trend amongst milkmaids?

Prior exposure to smallpox.

Explanation:

The small pox is the infectious disease caused by the different strains of variola virus. The vaccination for the small pox is available to eradicate the disease.

The milkmaids are already and multiple times have the exposure with the small pox. This develops the memory of the small pox virus infection in the immune cells of the individual and they are less likely on the infection exposure with the small pox.

Thus, the answer is prior exposure to smallpox.