Re: who discovered that rats were the carrier of the bubonic plague?

Dear Brandy,
     To answer your first question:
Through the written history of plague dead rats have been associated with 
outbreaks, but it was not until 1894 that Alexandre Yersin identified the 
bacteria Yersinia pestis as the cause of plague.  In 1898 EH. Hawkin and 
P.L. Simond are credited with discovering the role of the rat, and 2 years 
later Simond also discovered the role of the flea.  
     The most famous plague epidemic, The Black Death, obviously ended long 
before scientists knew what caused it.  They did, however, know that it was 
being brought to Europe by ships, and so they quarantined ships in the 
harbor until they thought there was no danger.  Many communities also 
quarantined the sick away from the town.  These measures probably played a 
part in ending the epidemic.
     Another reason it came to an end could be that the bacteria did not 
evolve in humans, but instead in animals, and humans are only accidental 
hosts.  The plague came through, killed susceptible individuals, and left 
others with  immunity.  A bacteria that evolves in humans usually only 
makes the person sick, and then is spread to another.  But Y. pestis kills 
so quickly that once it has come in contact with every member of a 
community and either killed them or the person has survived and acquired 
immunity, the bacteria needs another community.  If it cannot get into 
another community quickly the epidemic can "burn out."
     Yet another theory is that the Y. pestis mutated to be less virulent 
(harmful.)  Since mortality is still 50% in untreated plague, this does not 
seem very likely.
     Plague is still alive and well, however, and its status is not 
epidemic at the present time, but pandemic, that is, it exists all over the 
world.  But public health measures keep it relatively under control.  There 
are appoximately 2000 cases a year worlwide, and there are even a few cases 
a year in the American Southwest!  There is also a vaccine for Y. Pestis 
now.

References:
Kupferschmidt, H. "Epidemiology of the plague. Changes in the concepts in 
research of infection chain since discovery of the pathogen in 1894.  
Gesnerus Supplement. 43:1-222, 1993.

Slack, Paul. "The black death past and present. Some historical problems. 
Transactions of the Royal Society of Tropical Medicine and Hygiene. 
83:461-463. 1989.

Titball, R.W., and S.E. Leary. "Plague."  British Medical Bulletin. 
54:625-33. 1998.

Thanks for your question, 
Sarah Martin Mason, Mad Scientist at Tulane School of Medicine