Rodent ectoparasites from the Middle-East were known to be the root of many different easily transmissible fatal diseases prior to the the European plagues. Jewish traders were accused of possessing this knowledge then spreading the plague throughout Europe (for the usual motives of capitalizing on the demise of others and taking over).

It was told that farmers living in the countryside of towns would see a black shrouded figure at night with face covered using a long rod with some sort of substance spraying from it on the main roads leading in and out of towns immediately before a town was infected. This figure is the origin of the western grim reaper we have come to know with scythe in hand. Those who saw the figure, not knowing what the purpose of the the staff was, surmised that the swinging it it back and forth resembled a scyth farm tool..

Not much has changed in 600+ years.