Yersinia pestis nearly conquered Europe in the fourteenth century with the
help of the flea. Yersinia pestisis the microorganism that caused the Black
Plague (Fig. 1-2) and killed more than 25 million Europeans. You might say that
Yersinia pestislaunched a sneak attack. First, it infected fleas that were carried
into populated areas on the backs of rats. Rodents traveled on ships and then
over land in search of food. Fleas jumped from rodents and bit people, trans-
mitting the Yersinia pestismicroorganism into the person’s blood stream.
In an effort to prevent the spread of Yersinia pestis, sailors entering Sicily’s
seaports had to wait 40 days before leaving the ship. This gave time for sailors
to exhibit the symptoms of the Black Plague if the Yersinia pestismicroorgan-
ism had infected them. Sicilians called this quarantenaria. Today we know it as
quarantine. Sailors who did not exhibit these symptoms were not infected and
free to disembark.
Campers and travelers sometimes become acquainted with Giardia lamblia,
Escherichia coli, or Entameba histolyticawhenever they visit tropical countries.
Travelers who become infected typically do not die but come down with a bad
case of diarrhea.
FRIENDLY MICROORGANISMS
Not all microorganisms are pathogens. In fact many microorganisms help to
maintain homeostasis in our bodies and are used in the production of food and
other commercial products. For example, floraare microorganisms found in
our intestines that assist in the digestion of food and play a critical role in the
formation of vitamins such as vitamin B and vitamin K. They help by breaking
down large molecules into smaller ones.
CHAPTER 1 The World of the Microorganism^3
Fig. 1-2. Yersinia pestis is the microorganism
that caused the Black Plague.