A historian would have a difficult time determining A. the impact of smallpox on the body.
While they may have access to historical accounts, statistics, and records regarding the percentage of deaths (B), the types of disease (C), and the connection between disease and the Spanish conquest (D), the specific physiological impact of smallpox on individuals in historical populations would be challenging to quantify due to a lack of direct evidence and the subjective nature of personal experiences. Medical knowledge and techniques available at the time, as well as the variation in individual responses to disease, further complicate this assessment.