Yersinia pestis

A tiny germ called *Yersinia pestis* is the cause of the plague. This bacterium is a very small living thing. It is shaped like a little rod, which scientists call a coccobacillus. It does not move on its own, and it does not make spores. This germ is a hyperparasite. This means it lives inside a parasite, like a flea, which lives inside another animal, like a rat.

The plague is a serious disease that can take three different forms. These forms are called bubonic, septicemic, and pneumonic. The pneumonic form is special because it can spread through the air. This happens through tiny droplets when someone coughs or sneezes.

There is a very interesting way this germ moves from animals to humans. It often uses the oriental rat flea as a way to travel. When a flea bites an infected animal, the bacteria begin to grow inside the flea. The bacteria build up a thick layer called a biofilm in the flea's throat. This build-up is so thick that it blocks the flea from eating. This blockage is sometimes called Bacot's block. When the hungry flea tries to feed again, it cannot swallow the blood. Instead, it spits the bacteria back into the wound of the host.

Scientists have worked hard to understand this germ for a long time. A doctor named Alexandre Yersin discovered the bacterium in 1894. He found it during a plague outbreak in Hong Kong. He worked at the Pasteur Institute and was part of the Pasteur school of thought. Another scientist named Kitasato Shibasaburō was also looking for the cause of the plague at that time. At first, the germ was given the name *Pasteurella pestis*. Later, in 1944, it was renamed *Yersinia pestis* to honor Yersin.

Even though the plague is old, it is still around today. The World Health Organization reports one thousand to two thousand cases every year. Most cases now happen in places like Madagascar, Peru, and the Democratic Republic of the Congo. In the past, the plague caused huge events like the Black Death. It also caused the Plague of Justinian. The germ can also make many other animals sick. For example, it can affect prairie dogs and the endangered black-footed ferret. In North America, squirrels and chipmunks can also carry the germ.

Today, we have much better ways to fight the plague than people did in the past. Doctors use medicines called antibiotics to treat people who get sick. These medicines help the body fight off the bacteria. We also know more about the germ's tiny parts. Scientists have studied its genome, which is the complete set of instructions for the living thing. They have even looked at its proteins to see how it works. Understanding these small details helps us keep people and animals safe from the disease.