Yersinia pestis

Tiny germs can cause a sickness called the plague.

These germs often live inside small fleas. The fleas live on animals like rats.

A flea can bite a person. This can pass the germs to them. The germs can also spread through the air.

Many animals can get sick from these germs. This includes squirrels and prairie dogs.

Doctors have medicine to help people now. This medicine works much better than before. It is good that we have help.

A tiny germ causes the plague. We call this germ *Yersinia pestis*. It is a type of bacterium. Bacteria are very small living things.

These germs often live inside fleas. The fleas live on animals like rats. When a flea bites a person, it can pass the germ along. This happens because the germs build up in the flea. This build-up blocks the flea's throat. The flea tries to eat, but it spits the germs back into the wound. This is how people get sick.

The plague can also spread through the air. This happens when someone coughs or sneezes. Many animals can get sick from these germs too. This includes squirrels and prairie dogs.

In the past, the plague was very deadly. It caused big sicknesses like the Black Death. Today, doctors have medicine called antibiotics. These drugs help people get better. There are still one thousand to two thousand cases every year. Most cases are now in places like Madagascar and Peru.

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.

*Yersinia pestis* is a gram-negative bacterium that causes the disease known as the plague. This organism is a non-motile coccobacillus, which means it is a small, rod-shaped cell that cannot move on its own. It is also a facultative anaerobe, meaning it can survive with or without oxygen. *Y. pestis* is classified as a hyperparasite. This is because it lives inside a parasite, such as a flea, which is itself a parasite of a host, such as a rodent. This complex relationship allows the bacterium to move between different species.

The bacteria cause three distinct clinical forms of the disease. The bubonic plague is the most well-known form. The septicemic plague occurs when the bacteria enter the bloodstream. The pneumonic plague is a version that affects the lungs. This form is particularly dangerous because it can spread through the air via respiratory droplets. If a person catches the pneumonic form, they can pass it to others through coughing or sneezing. This cycle of infection makes the pneumonic form a significant threat to public health.

The mechanism of transmission often involves the oriental rat flea, *Xenopsylla cheopis*. When a flea feeds on an infected animal, the bacteria begin to multiply in the flea's digestive tract. The bacteria produce proteins that lead to the formation of a biofilm. This biofilm creates a physical blockage in the proventriculus, which is a valve in the flea's throat. Scientists call this accumulation "Bacot's block." Because the flea cannot swallow blood, it attempts to feed more aggressively. During these attempts, the flea regurgitates the bacteria back into the host's wound. This process effectively injects the infection into the new host.

*Y. pestis* uses several advanced tools to survive inside a host. It possesses plasmids, which are small pieces of DNA that carry specific instructions. The plasmid pPCP1 codes for a protease called Pla. This protein is a virulence factor that helps the bacteria cause pneumonic plague. Another plasmid, pMT1, helps the bacteria survive within the flea. The bacteria also use a type-III secretion system, or T3SS. This is a molecular machine that allows the bacteria to inject proteins, called Yops, directly into host immune cells. These proteins help the bacteria avoid being destroyed by the host's immune system.

History shows that *Y. pestis* has shaped human civilization through massive pandemics. It was the cause of the Plague of Justinian and the Black Death. One of the deadliest events in recorded history was the Black Death. The bacterium was discovered in 1894 by Alexandre Yersin. He was a Swiss and French physician working during an epidemic in Hong Kong. At the time, Kitasato Shibasaburō was also searching for the cause of the plague. Yersin originally named the organism *Pasteurella pestis*. It was renamed *Yersinia pestis* in 1944 to honor his discovery.

While the plague is an ancient disease, it remains a modern concern. The World Health Organization reports between 1,000 and 2,000 cases every year. Today, the disease is most commonly found in Madagascar, Peru, and the Democratic Republic of the Congo. During the Vietnam War, cases in Asia increased five to sixfold. This increase was likely due to ecosystem disruptions and closer contact between people and animals. Fortunately, the prognosis for patients is much better now than in the past. This is because doctors can use antibiotic treatments to fight the infection effectively.

The bacterium also impacts the natural world and various animal populations. Over 200 species of rodents serve as reservoirs for the disease. In North America, common reservoirs include prairie dogs, chipmunks, and squirrels. In Asia, the bacteria are often found in gerbils and susliks. The plague can be devastating to certain species, such as the black-tailed prairie dog. In these colonies, the disease can cause a total colony collapse. This collapse can have a massive effect on the entire prairie food web.