3 types of plague


Plague is an infectious disease caused by the bacteria Yersinia pestis, a zoonotic bacteria, usually found in small mammals and their fleas. It is transmitted between animals through fleas. Humans can be infected through:

  • the bite of infected vector fleas
  • unprotected contact with infectious bodily fluids or contaminated materials
  • the inhalation of respiratory droplets/small particles from a patient with pneumonic plague.

Plague is a very severe disease in people, particularly in its septicaemic (systemic infection caused by circulating bacteria in bloodstream) and pneumonic forms, with a case-fatality ratio of 30% to 100% if left untreated. The pneumonic form is invariably fatal unless treated early. It is especially contagious and can trigger severe epidemics through person-to-person contact via droplets in the air.

From 2010 to 2015, there were 3248 cases reported worldwide, including 584 deaths.

Historically, plague was responsible for widespread pandemics with high mortality. It was known as the “Black Death” during the fourteenth century, causing more than 50 million deaths in Europe. Nowadays, plague is easily treated with antibiotics and the use of standard precautions to prevent acquiring infection.

Signs and symptoms

People infected with plague usually develop acute febrile disease with other non-specific systemic symptoms after an incubation period of one to seven days, such as sudden onset of fever, chills, head and body aches, and weakness, vomiting and nausea.

There are two main forms of plague infection, depending on the route of infection: bubonic and pneumonic.

  • Bubonic plague is the most common form of plague and is caused by the bite of an infected flea. Plague bacillus, Y. pestis, enters at the bite and travels through the lymphatic system to the nearest lymph node where it replicates itself. The lymph node then becomes inflamed, tense and painful, and is called a ‘bubo’. At advanced stages of the infection the inflamed lymph nodes can turn into open sores filled with pus. Human to human transmission of bubonic plague is rare. Bubonic plague can advance and spread to the lungs, which is the more severe type of plague called pneumonic plague.
  • Pneumonic plague, or lung-based plague, is the most virulent form of plague. Incubation can be as short as 24 hours. Any person with pneumonic plague may transmit the disease via droplets to other humans. Untreated pneumonic plague, if not diagnosed and treated early, can be fatal. However, recovery rates are high if detected and treated in time (within 24 hours of onset of symptoms).

Where is plague found?

As an animal disease, plague is found in all continents, except Oceania. There is a risk of human plague wherever the presence of plague natural foci (the bacteria, an animal reservoir and a vector) and human population co-exist.

Plague epidemics have occurred in Africa, Asia, and South America; but since the 1990s, most human cases have occurred in Africa. The three most endemic countries are the Democratic Republic of Congo, Madagascar, and Peru. In Madagascar cases of bubonic plague are reported nearly every year, during the epidemic season (between September and April).

Diagnosing plague

Confirmation of plague requires lab testing. The best practice is to identify Y. pestis from a sample of pus from a bubo, blood or sputum. A specific Y. pestis antigen can be detected by different techniques. One of them is a laboratory validated rapid dipstick test now widely used in Africa and South America, with the support of WHO.


Untreated pneumonic plague can be rapidly fatal, so early diagnosis and treatment is essential for survival and reduction of complications. Antibiotics and supportive therapy are effective against plague if patients are diagnosed in time. Pneumonic plague can be fatal within 18 to 24 hours of disease onset if left untreated, but common antibiotics for enterobacteria (gram negative rods) can effectively cure the disease if they are delivered early.


Preventive measures include informing people when zoonotic plague is present in their environment and advising them to take precautions against flea bites and not to handle animal carcasses. Generally people should be advised to avoid direct contact with infected body fluids and tissues. When handling potentially infected patients and collecting specimens, standard precautions should apply.


WHO does not recommend vaccination, expect for high-risk groups (such as laboratory personnel who are constantly exposed to the risk of contamination, and health care workers).

Managing plague outbreaks

  • Find and stop the source of infection. Identify the most likely source of infection in the area where the human case(s) was exposed, typically looking for clustered areas with large numbers of small animal deaths. Institute appropriate infection, prevention and control procedures. Institute vector control, then rodent control. Killing rodents before vectors will cause the fleas to jump to new hosts, this is to be avoided.
  • Protect health workers. Inform and train them on infection prevention and control. Workers in direct contact with pneumonic plague patients must wear standard precautions and receive a chemoprophylaxis with antibiotics for the duration of seven days or at least as long as they are exposed to infected patients.
  • Ensure correct treatment: Verify that patients are being given appropriate antibiotic treatment and that local supplies of antibiotics are adequate.
  • Isolate patients with pneumonic plague. Patients should be isolated so as not to infect others via air droplets. Providing masks for pneumonic patients can reduce spread.
  • Surveillance: identify and monitor close contacts of pneumonic plague patients and give them a seven-day chemoprophylaxis. Chemoprophylaxis should also be given to household members of bubonic plague patients.
  • Obtain specimens which should be carefully collected using appropriate infection, prevention and control procedures and sent to labs for testing.
  • Disinfection. Routine hand-washing is recommended with soap and water or use of alcohol hand rub. Larger areas can be disinfected using 10% of diluted household bleach (made fresh daily).
  • Ensure safe burial practices. Spraying of face/chest area of suspected pneumonic plague deaths should be discouraged. The area should be covered with a disinfectant-soaked cloth or absorbent material.

Surveillance and control

Surveillance and control requires investigating animal and flea species implicated in the plague cycle in the region and developing environmental management programmes to understand the natural zoonosis of the disease cycle and to limit spread. Active long-term surveillance of animal foci, coupled with a rapid response during animal outbreaks has successfully reduced numbers of human plague outbreaks.

In order to effectively and efficiently manage plague outbreaks it is crucial to have an informed and vigilant health care work force (and community) to quickly diagnose and manage patients with infection, to identify risk factors, to conduct ongoing surveillance, to control vectors and hosts, to confirm diagnosis with laboratory tests, and to communicate findings with appropriate authorities.

WHO Response

WHO aims to prevent plague outbreaks by maintaining surveillance and supporting at-risk countries to prepare. As the type of animal reservoir differs according to the region and influences the risk and conditions of human transmission, WHO has developed specific guidelines for the Indian sub-continent, South-America and Sub-Saharan Africa.

WHO works with ministries of health to support countries facing outbreaks for field control activities.

1. The Plague of Justinian

Painting showing the plague in Constantinople. (Credit: Walters Art Museum)

Justinian I is often credited as the most influential Byzantine emperor, but his reign also coincided with one of the first well-documented outbreaks of plague. The pandemic is believed to have originated in Africa and then spread to Europe through infected rats on merchant ships. It reached the Byzantine capital of Constantinople in 541 A.D., and was soon claiming up to 10,000 lives per day—so many that unburied bodies were eventually stacked inside buildings or left in the open. According to accounts by the ancient historian Procopius, the victims demonstrated many of the classic symptoms of bubonic plague, including sudden fever and swollen lymph nodes. Justinian himself was stricken and managed to recover, but over a third of Constantinople’s residents were not so lucky. Even after it subsided in Byzantium, the plague continued to reappear in Europe, Africa and Asia for several years, causing widespread famine and devastation. It is believed to have killed at least 25 million people, but the actual death toll may have been much higher.

2. The Black Death

Scene of the plague in Florence. (Credit: DeAgostini/Getty Images)

In 1347, a virulent strain of plague invaded Europe from the East, most likely via Italian sailors returning home from Crimea. This “Black Death” would eventually spend half a decade tearing across the continent. The populations of whole towns were wiped out, and it was said that the living spent most of their time burying the dead in mass graves. “We see death coming into our midst like black smoke,” the Welsh poet Jeuan Gethin wrote, “a plague which cuts off the young, a rootless phantom which has no mercy or fair countenance.” Medieval physicians tried to combat the disease using bloodletting, lancing and other crude techniques, but with little understanding of its cause, most fell back on the belief that it was a divine punishment for their sins. Some Christians even blamed it on Jews and launched bloody pogroms. The Black Death finally subsided in the West around 1353, but not before it killed as many as 50 million people—more than half the population of Europe. While the pandemic left much of the continent in disarray, many historians also believe that the labor shortages it caused were a boon to lower class workers, who saw increased economic and social mobility.

3. The Italian Plague of 1629-31

Drawing of the Great Plague in Milan. (Credit: Fine Art Images/Heritage Images/Getty Images)

Even after the Black Death ended, bubonic plague continued to sporadically rear its ugly head in Europe for several centuries. One of the most calamitous outbreaks began in 1629, when troops from the Thirty Years’ War carried the infection into the Italian city of Mantua. Over the next two years, the plague snaked its way across the countryside, striking the major cities of Verona, Milan, Venice, and Florence. In Milan and Venice, city authorities quarantined the sick in “pesthouses” and burned their clothes and possessions to prevent the spread of infection. The Venetians even banished some of their plague victims to a pair of islands in a nearby lagoon. These harsh measures may have helped contain the scourge, but it still killed some 280,000 people, including over half the residents of Verona. The Republic of Venice, meanwhile, lost nearly a third of its population of 140,000. Some scholars have since argued that the outbreak may have sapped the city-state’s strength and led to its decline as a major player on the world stage.

4. The Great Plague of London

Drawing by William Blake of plague victims. (Credit: Fine Art Images/Heritage Images/Getty Images)

Plague laid siege to the city of London several times during the 16th and 17th centuries, most famously between 1665 and 1666. The pestilence first arose in the suburb of St. Giles-in-the-Fields, but it soon traveled into the cramped and filthy neighborhoods of the city proper. At its peak in September 1665, some 8,000 people were dying each week. The wealthy—including King Charles II—fled to the countryside, leaving the poor as the plague’s main victims. “Never did so many husbands and wives die together,” a reverend named Thomas Vincent wrote, “never did so many parents carry their children with them to the grave.” As the sickness spread, London’s authorities tried to contain the infected by quarantining them in their homes, which were marked with a red cross. Somewhere between 75,000 and 100,000 people eventually perished before the outbreak died down in 1666. Later that same year, London was visited by a second major tragedy when the Great Fire of 1666 torched much of its city center.

5. The Great Plague of Marseille

Painting of Marseille during the plague. (Credit: Robert Valette/Wikimedia Commons)

Western Europe’s last major outbreak of medieval plague began in 1720, when a “mortal distemper” seized the French port city of Marseille. The disease arrived on a merchant ship called the Grand Saint Antoine, which had picked up infected passengers during a journey to the Middle East. The vessel was quarantined, but its owner—who also happened to be Marseille’s deputy mayor—convinced health officials to let him unload its cargo. Plague-carrying rat fleas soon spread across the city, sparking an epidemic. People died by the thousands, and the piles of bodies on the streets grew so large that convicts were conscripted to dispose of them. In nearby Provence, “plague walls” were even built to try to and contain the infection, but it still spilled over into southern France before finally disappearing in 1722. By then, it had killed roughly 100,000 people.

6. The Third Plague Pandemic

People in quarantine in Karachi during the outbreak. (Credit: Wellcome Library, London/Creative Commons CC BY 4.0)

The first two major plague pandemics began with the Plague of Justinian and the Black Death. The most recent, the so-called “Third Pandemic,” erupted in 1855 in the Chinese province of Yunnan. The disease traversed the globe over the next several decades, and by the beginning of the 20th century, infected rats traveling on steamships had carried it to all six inhabited continents. The worldwide outbreak would eventually claim some 15 million lives before petering out in the 1950s. Most of the devastation took place in China and India, but there were also scattered cases from South Africa to San Francisco. Despite the heavy casualties, the Third Pandemic led to several breakthroughs in doctors’ understanding of the bubonic plague. In 1894, a Hong Kong-based doctor named Alexandre Yersin identified the bacillus Yersinia pestis as the cause of the disease. A few years later, another physician finally confirmed that bites from rat fleas were the main way the infection spread to humans.

Plague is often associated with the Middle Ages, but the infamous disease wreaked havoc before and after that time, and continues to infect people today. If left untreated, the bubonic plague can have a fatality rate of 50 to 60 percent, according to the World Health Organization. Antibiotics, developed in the 1940s, are effective in treating plague today.

Plague is found on every continent, but currently, plague is most prevalent in sub-Saharan Africa and Madagascar. More than 90 percent of current reported cases are found there, according to a review in PLOS Medicine. More than 1,000 cases of plague have been reported in the United States in the past 100 years.

Plague is more likely to occur in rural areas where people are exposed to wild rodents. It is more common in the rural Western United States than the East, though it is still rare.

What is plague?

Plague is an infection caused by an extremely virulent bacterium, Yersinia pestis, according to the Centers for Disease Control (CDC). Scientist Alexandre Yersin discovered Yersinia pestis in 1894. The bacterium is usually found in rodents and their fleas. Throughout history, urban rats have been the most dangerous plague-carriers to humans. Infected rat fleas can transmit Yersinia pestis to humans through their bites. Wild animals can catch plague through eating infected animals. This can sometimes lead to an outbreak among animals called an epizootic. Plague rates in humans tend to go up after epizootics, according to the CDC.

According to National Geographic, the virulence of Yersinia pestis is due to its ability to disable the host’s immune system. Yersinia pestisinjects toxins into defense cells, leading to the breakdown of the immune system. Then, the bacteria multiply rapidly, infecting the body.

A map of the 66 confirmed animal plague cases that occurred in the United States between 2000 and 2015. (Image credit: Walsh M. and Haseeb M.A., PeerJ, 2015. )

Types of plague

There are three types of plague and all begin with the same basic symptoms. According to the WHO, people with plague usually develop flu-like symptoms three to seven days after being bitten or otherwise infected. These symptoms include fever, chills, body aches, vomiting, nausea and weakness.

Bubonic plague is the most common plague type, according to the WHO. It is caused by the bite of an infected flea, often a rat flea. In addition to flu-like symptoms, patients’ lymph nodes become tender and swollen. The lymph nodes can become visibly inflamed and quite large. The inflamed lymph nodes are called “buboes,” which given the plague its name. When bubonic plague advances, the buboes can become festering open sores.

When Yersinia pestis enters the body, it travels to the nearest lymph node, shuts down the defenses, and replicates itself. This is the first lymph node to become a bubo. If patients are not treated promptly, bubonic plague can spread to other lymph nodes. Bubonic plague rarely spreads from person to person.

Septicemic plague is plague of the blood. It can come from flea bites or direct contact with an infected animal, if the infected materials enter through cracks in the skin. It can also develop from untreated advanced bubonic plague, according to the CDC. Yersinia pestis enters the bloodstream and multiplies there.

In addition to flu-like symptoms, patients with septicemic plague experience extreme weakness, shock and abdominal pain. There can be internal bleeding that often causes skin and other tissues to turn black and die. This necrosis is most often seen on the nose, fingers and toes.

Pneumonic plague is the most deadly form of plague and the only one that can spread from person to person, according to the CDC. Pneumonic plague infects the lungs and can be transmitted through coughs. Occasionally, people get it from the coughs of their pet cats, which are susceptible to plague. Pneumonic plague can also develop from advanced, untreated bubonic plague that spreads to the lungs.

Pneumonic plague causes patients to develop serious pneumonia. Symptoms include fever, chills, weakness, a rapidly developing cough, shortness of breath, chest pain and watery or bloody mucus. It can cause respiratory failure or shock.

Fortunately, pneumonic plague is the least common form of plague, according to the WHO.

This 1977 image shows the necrotic flesh that gave the Black Death its name. This symptom is known as acral gangrene. (Image credit: CDC/ William Archibald)

Diagnosis and treatment

If a healthcare worker suspects plague, he or she will sample the patient’s blood, sputum or lymph node aspirate and send them for lab tests, according to the CDC. Preliminary results can be ready in less than two hours. Confirmation can take 24 to 48 hours.

Plague is treated with readily available antibiotics. Often, antibiotics are given as soon as samples are taken because the sooner the patient begins treatment the better the chances for full recovery. If the patient has pneumonic plague, people in close contact with him or her may be evaluated, placed under observation and given preventative antibiotics, according to the CDC.

Various plague vaccines have been developed but their effectiveness has been inconclusive and they are no longer available in the United States, according to the CDC.

History of plague

There have been three major plague epidemics throughout human history. According to a history of plague published in Baylor University Medical Center Proceedings, the earliest instance of plague was probably recounted in the Bible. The First Book of Samuel accounts that around 1000 B.C., the Philistines were afflicted with a terrible disease involving swollen lymph nodes.

The Justinian Plague, however, was the first epidemic to be reliably recorded, according to Susan Abernethy, a Colorado-based historian and writer.

Justinian Plague

The Justinian Plague took place from approximately A.D. 542 until 750. It began during the reign of Justinian I, a Byzantine emperor based in Constantinople.

“The origin of the plague is unknown and there is little information available on how often and where the disease broke out in the following centuries,” said Abernethy. Though there are no reliable numbers regarding deaths, there was a significant drop in population. The Byzantine Empire and surrounding Mediterranean areas may have experienced as much as a 40 percent loss in population during the latter half of the sixth century.

The losses in population created worker shortages and a reduced tax base. Labor costs and inflation increased while food production decreased, leading to additional deaths by starvation, Abernethy told Live Science.

The Justinian Plague had a significant effect on European culture, said Robert Wilde, a UK-based historian and writer. At the time of the plague outbreak, the Eastern part of the Roman Empire (Byzantium), was much stronger culturally and militarily than the Western part, which had been without an emperor for some time. “Emperor Justinian had overseen a re-conquest of large areas of the old Western empire. But the plague destroyed these efforts, and weakened Byzantium trade, economy, military and society so much they were reduced forever in size,” Wilde said. Without this plague, Byzantine culture and the Roman Empire might have existed for much longer.

The Black Death

The Black Death occurred throughout Europe during the 14th ;century and killed approximately 25 million people. Bubonic plague spread through rats and fleas, while pneumonic plague spread from person to person. Europe lost between 33 and 50 percent of its population, according to Wilde.

The Black Death originated in China in 1334 and spread west along the trade routes of the Near and Far East, said Abernethy. By the early 1340s, the disease had struck China, India, Persia, Syria and Egypt. Many Europeans heard rumors of a “Great Pestilence” that was making its way across these routes.

“The plague arrived in Europe by sea in October of 1347 when 12 Genoese trading ships docked in the Sicilian port of Messina following a long journey through the Black Sea,” said Abernethy. “People gathered on the docks to greet the ships and were horrified to find most of the sailors aboard were either dead or gravely ill. The men were burning with fever, unable to keep food down and delirious with pain. Strangest of all, they were covered with mysterious black boils, which oozed blood and pus. The illness came to be known as the Black Death as a result.”

European leaders had no knowledge on how to contain outbreaks of disease. The Sicilian authorities quickly ordered the ships out of the harbor but it was too late. The disease quickly spread.

The Black Death changed Europe’s economy and wealth distribution. The loss of population resulted in larger inheritances, concentrating the wealth. At the same time, wages increased because of greater demand. Rich landowners turned to technology to save money. According to Wilde, the greater wealth concentration was “a massive cause of the Reformation, when money, power and art directly intersected.”

Wilde added, “In many ways the Black Death triggered the start of the evolution of medieval society into modern, but I think it’s important to stress the massive psychological impact these losses had on the survivors, as borne out by much northern art.”

The Modern Plague, or Third Pandemic

The Modern Plague began in China’s Yunnan province in 1855, said Abernethy, and “killed more than 12 million people in India and China alone.”

There were two strains of plague during the Third Pandemic. A bubonic strain spread through transportation of cargo, people and rats across the oceans. A more virulent pneumonic strain was largely confined to Manchuria and Mangolia, Abernethy said.

According to Abernethy, a notable characteristic of the Modern Plague is the amount of research that came out of it. “Scientists working in Asia during the outbreak identified plague carriers and the plague bacillus. Alexandre Yersin, working in Hong Kong in 1894, identified Yersinia pestis … In 1898, French researcher Paul-Louis Simond confirmed the role of fleas as the transporter of the disease. This plague is also more documented than the earlier pandemics.”

Chemical warfare

Plague has been used as a weapon of war throughout history, and national security officials continue to worry about its use as a biological weapon. According to a history published in the journal Emerging Infectious Diseases, there are first-hand accounts of Mongol armies catapulting bubonic plague-carrying corpses over the city walls of Caffa, a city in Crimea, in the 1300s. Some scholars believe that this tactic contributed to the Black Death.

The Japanese army experimented on plague and is reported to have dropped plague-infected fleas on areas of China and Manchuria during World War II, according to Baylor University Medical Proceedings. During the Cold War, many countries, including the United States and the Soviet Union, researched plague as a biological weapon, though neither utilized it. After the attacks on September 11, national security officials again began to worry about the threat of bioterrorism, including plague.

According to Johns Hopkins University, a weaponized plague outbreak would look different from a naturally occurring pandemic. The bacteria would likely be released as an aerosol, and the first sign of the attack would be a sudden outbreak. Cases would appear one or two days after the attack and people would die quickly. A 1970 WHO analysis of a worst-case scenario showed aerosol released over a city of 5 million resulting in 150,000 cases of pneumonic plague and 36,000 deaths.

Additional resources

  • CDC: Plague
  • JAMA: Plague as a Biological Weapon
  • PLOS: Plague: Past, Present and Future

Frequently Asked Questions

Plague is an infectious disease that affects rodents, certain other animals, and humans. It is caused by the Yersinia pestis bacteria. These bacteria are found in many areas of the world, including the United States.

How do people become infected with plague?

People most commonly acquire plague when they are bitten by a flea that is infected with the plague bacteria. People can also become infected from direct contact with infected tissues or fluids while handling an animal that is sick with or that has died from plague. Finally, people can become infected from inhaling respiratory droplets after close contact with cats and humans with pneumonic plague.

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What are the different forms of plague?

There are three forms of plague:

  • Bubonic plague: Patients develop sudden onset of fever, headache, chills, and weakness and one or more swollen, tender and painful lymph nodes (called buboes). This form is usually the result of an infected flea bite. The bacteria multiply in the lymph node closest to where the bacteria entered the human body. If the patient is not treated with appropriate antibiotics, the bacteria can spread to other parts of the body.
  • Septicemic plague: Patients develop fever, chills, extreme weakness, abdominal pain, shock, and possibly bleeding into the skin and other organs. Skin and other tissues may turn black and die, especially on fingers, toes, and the nose. Septicemic plague can occur as the first symptoms of plague, or may develop from untreated bubonic plague. This form results from bites of infected fleas or from handling an infected animal.
  • Pneumonic plague: Patients develop fever, headache, weakness, and a rapidly developing pneumonia with shortness of breath, chest pain, cough, and sometimes bloody or watery mucous. Pneumonic plague may develop from inhaling infectious droplets or from untreated bubonic or septicemic plague that spreads to the lungs. The pneumonia may cause respiratory failure and shock. Pneumonic plague is the most serious form of the disease and is the only form of plague that can be spread from person to person (by infectious droplets).

Forms of plague.

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What is the basic transmission cycle of plague?

Fleas become infected by feeding on rodents, such as chipmunks, prairie dogs, ground squirrels, mice, and other mammals that are infected with the bacterium Yersinia pestis. Fleas transmit the plague bacteria to humans and other mammals during a subsequent feeding. The plague bacteria survive briefly (a few days) in the blood of rodents and for longer periods in the fleas. An illustration of plague ecology in the United States is available.

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Could one person get plague from another person?

Yes, when a person has plague pneumonia they may cough droplets containing the plague bacteria into air. If these bacteria-containing droplets are breathed in by another person they can cause pneumonic plague. Human-to-human transmission is rare and typically requires direct and close contact with the person with pneumonic plague.

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What is the incubation period for plague?

A person usually becomes ill with bubonic plague 2 to 6 days after being infected. Someone exposed to Yersinia pestis through the air would become ill within 1 to 3 days.

When bubonic plague is left untreated, plague bacteria can invade the bloodstream. When plague bacteria multiply in the bloodstream, they spread rapidly throughout the body and cause a severe and often fatal condition called septicemic plague. Untreated bubonic plague can also progress into an infection of the lungs, causing pneumonic plague. If plague patients are not given specific antibiotic therapy, all forms of plague can progress rapidly to death.

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How is plague diagnosed?

The first step in plague diagnosis is evaluation by a health worker. If the health worker suspects plague, samples of the patient’s blood, sputum, or lymph node aspirate are sent to a laboratory for testing. Once the laboratory receives the sample, preliminary results can be ready in less than two hours. Laboratory confirmation will take longer, usually 24 to 48 hours. Often, presumptive treatment with antibiotics will start as soon as samples are taken, if plague is suspected.

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How many cases of plague occur in the United States? Globally?

Plague was first introduced into the United States in 1900. Between 1900 and 2012, 1006 confirmed or probable human plague cases occurred in the United States. Over 80% of United States plague cases have been the bubonic form. In recent decades, an average of 7 human plague cases are reported each year (range: 1-17 cases per year). Plague has occurred in people of all ages (infants up to age 96), though 50% of cases occur in people ages 12–45. Worldwide, between 1,000 and 2,000 cases each year are reported to the World Health Organizationexternal icon (WHO), though the true number is likely much higher.

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What is the death rate of plague?

In the pre-antibiotic era (1900 through 1941), mortality among those infected with plague in the United States was 66%. Antibiotics greatly reduced mortality, and by 1990-2010 overall mortality had decreased to 11%. Plague can still be fatal despite effective antibiotics, though it is lower for bubonic plague cases than for septicemic or pneumonic plague cases. It is hard to assess the mortality rate of plague in developing countries, as relatively few cases are reliably diagnosed and reported to health authorities. WHO cites mortality rates of 8–10%, however some studies (WHO, 2004) suggest that mortality may be much higher in some plague endemic areas.

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How is plague treated?

Plague can be successfully treated with antibiotics. Once a patient is diagnosed with suspected plague they should be hospitalized and, in the case of pneumonic plague, medically isolated. Laboratory tests should be done, including blood cultures for plague bacteria and microscopic examination of lymph node, blood, and sputum samples. Antibiotic treatment should begin as soon as possible after laboratory specimens are taken. To prevent a high risk of death in patients with pneumonic plague, antibiotics should be given as soon as possible, preferably within 24 hours of the first symptoms.

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Is the disease seasonal in its occurrence?

Yes. Although cases can occur any time of the year, most cases in the United States are acquired from late spring to early fall.

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Where is plague most common in the United States?

Plague occurs in rural and semi-rural areas of the western United States. Plague is most common in the southwestern states, particularly New Mexico, Arizona, and Colorado.

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Who is at risk for getting plague in the United States?

Human plague occurs in areas where the bacteria are present in wild rodent populations. The risks are generally highest in rural and semi-rural areas, including homes that provide food and shelter for various ground squirrels, chipmunks and wood rats, or other areas where you may encounter rodents.

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Is a vaccine available to prevent plague?

A plague vaccine is not available. New plague vaccines are in development but are not expected to be commercially available in the immediate future.

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Plague is caused by the bacteria Yersinia pestis. Rodents, such as rats, carry the disease. It is spread by their fleas.

People can get plague when they are bitten by a flea that carries the plague bacteria from an infected rodent. In rare cases, people get the disease when handling an infected animal.

Plague lung infection is called pneumonic plague. It can be spread from person to person. When someone with pneumonic plague coughs, tiny droplets carrying the bacteria move through the air. Anyone who breathes in these particles may catch the disease. An epidemic can be started this way.

In the Middle Ages in Europe, massive plague epidemics killed millions of people. Plague has not been eliminated. It can still be found in Africa, Asia, and South America.

Today, plague is rare in the United States. But it has been known to occur in parts of California, Arizona, Colorado, and New Mexico.

The three most common forms of plague are:

  • Bubonic plague, an infection of the lymph nodes
  • Pneumonic plague, an infection of the lungs
  • Septicemic plague, an infection of the blood

The time between being infected and developing symptoms is typically 2 to 8 days. But the time can be as short as 1 day for pneumonic plague.

Risk factors for plague include a recent flea bite and exposure to rodents, especially rabbits, squirrels, or prairie dogs, or scratches or bites from infected domestic cats.

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