Complex sentence: Because they had no immunity to this highly contagious and lethal disease, the Manchus were particularly fearful of it: the Shunzhi emperor died of smallpox and his successor Kangxi was chosen because he had already survived the disease.
Split into simpler sentences: Because they had no immunity to this highly contagious and lethal disease, the Manchus were particularly fearful of it. The Shunzhi emperor died of smallpox and his successor Kangxi was chosen because he had already survived the disease.