Final answer:
Yang originally became sick likely due to the spread of an epidemic within the army during the Mongol invasion of Sichuan in 1258-59, aggravated by the conditions of siege warfare, movements of troops into new territories, and potentially infectious diseases including plague.
Step-by-step explanation:
Throughout the various accounts of the Mongol invasion in China, it is evident that epidemics played a significant part in the hardships faced by both the invading and local populations. Historical records from the Yuan dynasty indicated that a great epidemic occurred within the army camped at Mt. Diaoyu in Hezhou during the summer of 1259. The epidemic was severe enough that special medicinal preparations like medicated leaven were ordered to treat the afflicted soldiers. This suggests that Yang likely became sick due to vast movements of troops into unfamiliar territory which facilitated disease outbreaks. The nature of siege warfare further stressed the populations, resulting in additional health challenges such as starvation, overeating, and exposure to the elements, leading to weakened bodily defenses and, ultimately, illness. Multiple sources describe the epidemics as something new, resistant to the usual treatments, and connected to the movements of siege and war.
Moreover, references to similar circumstances in other regions during different periods, like the plight faced by Wang Mang's regime due to environmental catastrophes followed by disease and the advice to take rhubarb during epidemics, highlight the ongoing struggle with illness during historical military conflicts. High mortality rates reported in various cities during these times further suggest that the illnesses could be attributed to infectious diseases, possibly including plague, as highlighted by modern retrospection techniques.