/u/eyelastic

This was mentioned in The Diary of a Manchu Soldier in 17-th Century China, originally written by Dzengseo and translated by Nicola Di Cosmo, Disease and Its Impact on Politics, Diplomacy, and the Military: The Case of Smallpox and the Manchus, by C-F. Chang, in the Journal of the History of Medicine & Allied Sciences, 2002, and China Marches West, by Peter Perdue.

Prior to their conquest, the Manchus had limited exposure to small pox (and other diseases like malaria), due to a combination of their nomadic lifestyle, low population density, and limited direct contact with China. The Mings were used to limiting contact with the Manchus due to security reasons.

In densely populated parts of China, a lot of the infection happened to children, while among the Manchus it largely infected adults. This can lead to significant destabilization when important persons fall ill. So the Manchus were very, very fearful of smallpox.

Among the measures they took was quarantine. Very, very severe quarantine. A person found to have smallpox will be abandoned in a cave or distant outdoor, either they die of smallpox or survive and return. No exceptions would be made.

Over time, they started to favor appointment of persons who had survived smallpox as a child. One great example is the election of the Kangxi Emperor, who bore scars from the disease he survived. Important persons who had not had smallpox were exempted from having to visit the Emperor in Beijing in person.

Finally, under the Kangxi Emperor, an inoculation program of the royal family was started, and then expanded to include children of important leaders.

Smallpox continued to be a major threat, especially among Mongols with historically little contact with the Han Chinese, well into the twilight of the Qing dynasty. This problem became worse as contact with Han Chinese increased.