Map showing the spread of the Black Plague as well as the infected areas.
(Black Rat) Black rats helped to spread the black plague when infected traders brought them over on ships.
(flea) The flea was the original carrier of the bacteria that caused the plague. Rats spread the fleas and thus spreading the plague.
At least 25 million people died from the Black Plague (around 1/3 of the European population). The death rate was 12%-50% for infected people. Not everyone who got it died but many did.