The correct answer is Malaria
Malaria was a huge problem since it couldn't be treated and people mostly died from it. It prevented numerous explorers from venturing into places like Africa where Malaria was running rampant due to the huge amount of mosquitoes. When it was discovered, it enabled numerous people to live at ease because they could not live with the disease or prevent getting it.
Considering that the Qin Dynasty was the 1st imperial dynasty of China, it would definitely be that **A civil war broke out** (since there was a more large power vacuum to be filled.)