by Erich Rosenberger M.D.

Created on: January 18, 2008   Last Updated: March 31, 2008

In the 14th Century a deadly plague swept through Europe. Although the cause wasn't known at the time, this deadly infection was responsible for the death of more than a third of all people in Europe. It even reached as far as China, where it was equally as devastating. The final death count is debatable, but it is generally believed to be in the millions. The political, economic and social implications of The Black Plague were felt for hundreds of years.

It is now known that the cause of this plague was a tiny bacterium called yersina pestis. And even more tragic it can be easily treated with common antibiotics. Imagine how many lives were lost for the want of a little tablet.

Yersinia pestis is a small, rod-shaped bacteria. It does not stain with a common technique known as Gram staining. This is important in differentiating it from other bacteria, as these little critters are too small to just look and and tell one from another like you would a car or a tree, even with a microscope.

Yersina pestis is one of the most virulent bacteria known to man. It generally only takes between 1 and 10 individual bacteria to cause an active infection. That's really not a lot.

In America, the major source of the yersinia pestis bacteria is prairie dogs. Yes, they may be cute, but they carry a deadly disease. If you find yourself in a desert in the American Southwest, resist all temptation to cuddle with the wild prairie dogs.

During the Plague of the 14th Century and even in some parts of the world today, the major method of transmission is via rats and fleas. The path of infection from rats to humans is a very interesting story. The rat is the reservoir for the bacteria. Rats carry the bacteria around without being effected by it themselves.

Along comes a flea, who's role is to bite the rat and feed on its blood. Of course, the flea is in for a nasty surprise as it takes its blood meal, it gets a mouthful of bacteria as well. The bacteria makes a protein, called coagulase, which causes the blood to coagulate in the stomach of the flea.

The bacteria will then become trapped in this clot in the stomach of the flea. While there, the bacteria will multiply.

This mass of clotted blood and multiplying bacteria will not be properly digested by the flea. It will cause a blockage of the normal digestive process in the flea. The flea gets no nutritional value from the blood it sucked out of the rat. This makes it very hungry. It quickly looses it's preference for biting only rats and will

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