The bacterial cause of the bubonic plague which in the year 541 (as the black death) and later in the middle ages decimated europe. The effects of the plague are described in the nursery rhyme we all fall down. It is transmitted to humans by the bite of fleas that have fed on infected animals, mostly rodents. Plague occurs in the u.s. It is treatable with antibiotics but, if not treated promptly, can promptly lead to death.