Unlike flea-bearing rodents, which can actually contract bubonic plague, the fleas themselves are merely carriers of the disease.

Certain types of plague can pass directly from one human being to another, without flea or rodent intermediaries.

The plague-infected rodent species that were common in central Asia did not spread to Europe during the Middle Ages.

Plague bacilli were spread from central Asia to Europe by rats inadvertently transported along with tradable goods.

The smell of horses, which were a central feature of life among the nomadic peoples of central Asia, is repellent to the fleas that live on rodents.