Why did milkmaids feel off colour for a few days?

Milkmaids often felt off color for a few days due to exposure to cowpox, a virus that typically affected cows but could be transmitted to humans. This mild illness resulted in symptoms such as fever and discomfort, which were generally short-lived. The exposure was significant because it later led to the development of immunity against smallpox, as Edward Jenner's work demonstrated in the late 18th century. Thus, while milkmaids experienced temporary illness, it ultimately contributed to a broader public health benefit.