What did Qin Shi Huang die from?
Qin Terracotta Army
People who support that he died of an illness believe he suffered from overwork. Every day, during his reign, he had to scrutinize a large number of documents and attend to every petty detail personally to ensure the preservation of imperial power. Being the first emperor in Chinese history and the builder of a centralized system, Qin Shi Huang had no precedent to follow but still had full responsibility. Overwork brought on a serious illness to this industrious emperor. In addition he went on five big tours in 12 years. Even taking the most advanced highway "Chi Dao" of the time, he suffered badly from being jolted on the bumpy roads all the time. The luxurious chariot he took cannot be compared with the car, bus and train of the modern world. Therefore, the emperor must have gone through great hardship in his long royal procession. His final trip started during a severe winter and ended in hot summer - the great change of weather might have increased his chance of disease. Also he had more than 10,000 concubines and probably was frequently active in bed, which might have damaged his health.
Hu Hai's involvement is doubtful. He was the son of Qin Shi Huang. After learning of his brother Fu Su's succession to the throne, he might have felt dissatisfied and killed his father to vent his anger. But this presumption cannot stand firm because the rescript and royal seal were kept by Zhao Gao, whom Qin Shi Huang trusted most when he was ill. Without Zhao Gao's cooperation, Hu Hai still could not ascend to the throne even though he killed his father. Zhao was always by the side of Qin Shi Huang, therefore had more opportunities to kill the emperor. Li Si, the prime minister of Qin Dynasty, was also more likely to be the murderer, for he could keep his official position by hailing Hu Hai a new emperor after the old emperor died.
In folk legend, Emperor Qin Shi Huang died of a people's curse due to his tyrannical government. Of course, this is the most absurd hypothesis only showing the people's hatred of the emperor. Though the assertions are all logical they are unproved by any historical record or unearthed cultural relics. The death of the first emperor still remains a matter of question.
Further Reading: Why the Emperor Qin Shi Huang did not determine the empress?