Read this excerpt from "Edward Jenner and the History of Smallpox and Vaccination.”

The origin of smallpox as a natural disease is lost in prehistory. It is believed to have appeared around 10,000 BC, at the time of the first agricultural settlements in northeastern Africa. It seems plausible that it spread from there to India by means of ancient Egyptian merchants.

PLEASE HELP
What is the central claim of the excerpt?

It is difficult to trace the beginnings of smallpox.
Smallpox was most dangerous in prehistoric times.
Smallpox most likely originated from farms in Africa.
It is unclear how other diseases are related to smallpox.