Alrighty this is gonna be hard (free 50 points and brainliest if you get it right):At

Zheng He was a Chinese explorer who lead seven great voyages on behalf of the Chinese emperor. These voyages traveled through the South China Sea, Indian Ocean, Arabian Sea, Red Sea, and along the east coast of Africa. His seven total voyages were diplomatic, military, and trading ventures, and lasted from 1405 – 1433

There are a lot of factors that led up to this. One is that religious institutions used to block medical research as they felt it betrayed their core beliefs. We also subscribed to many psuedo-scientifical ideas that hindered our understandings of vaccinations and antibiotics for years. It also just takes a lot of time and a dedicated and educated society to procure these advancements.