This is an article prepared by unesco about diseases along the silk roads, precisely during 500 bc to 1500 ac.
https://en.unesco.org/silkroad/content/spread-disease-along-silk-roads-smallpoxI completely understand the concern about the lack of written evidence for variolation in the 15th century. However, it is important to note that our understanding of history and the spread of disease during that time is not limited to written records alone.
To gain a better understanding of this issue, let's take a look at what was happening during the period from 1000-1400 AC. Ancient civilizations were not primitive, and there was no single point in time where everything was invented overnight. There were differences between people of different time periods, mainly in terms of education, the development of processes, and the creation of structures for social well-being, especially around health.
It is well known that the variola virus affects not only humans but also animals. The animal version of the virus, commonly known as smallpox, can be transmitted quite easily to humans and can cause milder symptoms than the human version of the virus. Moreover, once someone contracts the human smallpox virus, they cannot be sick again from that same virus. If someone contracts the animal smallpox virus, they are unlikely to get sick upon contact with the human smallpox virus or develop much milder symptoms.
In any given community, people will eventually discover the link between contracting the animal smallpox virus and immunity to the human smallpox virus. This knowledge would have been passed down through generations, shared between different civilizations, and likely would have spread along trade routes such as the Silk Road.
It's true that the absence of written evidence doesn't necessarily mean that the knowledge wasn't present in the society. In fact, the absence of written evidence could also mean that the knowledge was so ingrained in the culture that it didn't require documentation. When we do find written evidence, however, it indicates that the knowledge was so widespread that someone felt compelled to write it down and share it with others.
The links I provided do suggest that the knowledge of inoculation or variolation was considered to be "thousands of years old." This reinforces the idea that the knowledge was deeply embedded in the society and had been passed down through generations. It's worth noting that not all knowledge was written down in ancient times. In many cases, knowledge was passed down orally from one generation to the next.
While it's true that we don't have the luxury of having every single piece of parchment or paper ever written, we can still draw conclusions based on the evidence that does exist. For example, if we look at the historical accounts of fathers making infected wool bracelets for their children, we can infer that this was a common practice among people who were knowledgeable about variolation. It's also likely that the knowledge of variolation was widely known because of the importance of maintaining healthy herds of farm animals.
The fact that this knowledge was so deeply embedded in the society is what made it possible for people to share it without fear of being labeled as charlatans or cultural appropriators. It's also what made it possible for the knowledge to survive over time, even without written evidence.