Respuesta :

it was Trade between Asia and Europe because it was caused by inflected fleas on rats that scurried between Asia and Europe and they weren't used to the plague.
Greenleafable
The correct answer is trade between Asia and Europe

Vermin such as rats which had the virus in Asia would board merchant ships and would get taken back to Europe. The voyages were long so the vermin would just keep spreading on the ship at first and then later in the country where the boat goas.