Cowpox can prevent smallpox because the cowpox virus has the same antigenic nature as the smallpox virus that causes smallpox disease in humans, and after a person is inoculated with the cowpox vaccine, he or she can also acquire immunity against the smallpox virus at the same time.

In fact, cowpox is not an agent, but a virus. Cowpox virus is a virus that causes mild cowpox lesions in cattle. A person infected with this virus will only experience mild discomfort and develop resistance to the cowpox virus.

Expanded information:

Generally, it is believed that smallpox was introduced during the Han Dynasty by prisoners captured in conquests The disease was introduced into China by prisoners captured during conquests during the Han Dynasty. Therefore, the disease was also called captive sores. According to Ge Hong's "After Elbow to Save Pawns", smallpox was "obtained from the captives in Nanyang during the Jianwu period, and it was almost a captive's sore". However, some scholars do not agree with this statement, pointing out that the word "Nanyang" in Ge Hong's "Elbow After Salvation for Pawns" is a clerical error. It actually refers to Jiao-toe, the southern border of China, where Liu Xiu, the Han emperor, sent his general Ma Yuan to fight.

In the "elbow to save the pawn formula", also recorded on the description of smallpox, which allows us to clearly see the harm of this disease: "than the years of illness when the line, it is hair sores on the head, face and body, a moment around, like a fire wound, are loaded with white pulp, with the decision to be born with, do not treat, the drama of the more dead. Cure the poor, the sore purpura purple and black, Miyu Fang extinguished, this evil gas."

Scientific Ningxia - human pox inoculation method: the first artillery against smallpox