Plague is an infection caused by the bacterium Yersinia pestis. The bacteria are found mainly in rats and in the fleas that feed on them. People and other animals can get plague from rat or flea bites. In the past, plague destroyed entire civilizations. Today plague is uncommon, due to better living conditions and antibiotics.

There are three forms of plague:

• Bubonic plague causes the tonsils, adenoids, spleen, and thymus to become inflamed. Symptoms include fever, aches, chills, and tender lymph glands.
• In septicemic plague, bacteria multiply in the blood. It causes fever, chills, shock, and bleeding under the skin or other organs.
• Pneumonic plague is the most serious form. Bacteria enter the lungs and cause pneumonia. People with the infection can spread this form to others. This type could be a bioterror agent.

Lab tests can diagnose plague. Treatment is a strong antibiotic. There is no vaccine.

Plague is a disease that affects humans and other mammals. It is caused by the bacterium, Yersinia pestis. Humans usually get plague after being bitten by a rodent flea that is carrying the plague bacterium or by handling an animal infected with plague. Plague is infamous for killing millions of people in Europe during the Middle Ages. Today, modern antibiotics are effective in treating plague. Without prompt treatment, the disease can cause serious illness or death. Presently, human plague infections continue to occur in rural areas in the western United States, but significantly more cases occur in parts of Africa and Asia.

The bacteria that cause plague, Yersinia pestis, maintain their existence in a cycle involving rodents and their fleas. Plague occurs in rural and semi-rural areas of the western United States, primarily in semi-arid upland forests and grasslands where many types of rodent species can be involved. Many types of animals, such as rock squirrels, wood rats, ground squirrels, prairie dogs, chipmunks, mice, voles, and rabbits can be affected by plague. Wild carnivores can become infected by eating other infected animals.

Scientists think that plague bacteria circulate at low rates within populations of certain rodents without causing excessive rodent die-off. These infected animals and their fleas serve as long-term reservoirs for the bacteria. This is called the enzootic cycle.

Occasionally, other species become infected, causing an outbreak among animals, called an epizootic. Humans are usually more at risk during, or shortly after, a plague epizootic. Scientific studies have suggested that epizootics in the southwestern United States are more likely during cooler summers that follow wet winters. Epizootics are most likely in areas with multiple types of rodents living in high densities and in diverse habitats.

In parts of the developing world, plague can sometimes occur in urban areas with dense rat infestations. The last urban outbreak of rat-associated plague in the United States occurred in Los Angeles in 1924-1925.

Plague in the United States

Plague was first introduced into the United States in 1900, by rat–infested steamships that had sailed from affected areas, mostly from Asia. Epidemics occurred in port cities. The last urban plague epidemic in the United States occurred in Los Angeles from 1924 through 1925. Plague then spread from urban rats to rural rodent species, and became entrenched in many areas of the western United States. Since that time, plague has occurred as scattered cases in rural areas. Most human cases in the United States occur in two regions:

• Northern New Mexico, northern Arizona, and southern Colorado
• California, southern Oregon, and far western Nevada

Over 80% of United States plague cases have been the bubonic form. In recent decades, an average of seven human plague cases have been 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. It occurs in both men and women, though historically is slightly more common among men, probably because of increased outdoor activities that put them at higher risk.

Reported Cases of Human Plague - United States, 1970-2019

Since the mid–20th century, plague in the United States has typically occurred in the rural West. The case shown in Illinois was lab-associated.

Between 1900 and 2012, 1006 confirmed or probable human plague cases occurred in the United States.

Plague Worldwide

Plague epidemics have occurred in Africa, Asia, and South America but most human cases since the 1990s have occurred in Africa. Almost all of the cases reported in the last 20 years have occurred among people living in small towns and villages or agricultural areas rather than in larger towns and cities.

Reported* Plague Cases by Country, 2013-2018

Plague is an infectious disease that affects animals and humans. It is caused by the bacterium Yersinia pestis. This bacterium is found in rodents and their fleas and occurs in many areas of the world, including the United States.

Y. pestis is easily destroyed by sunlight and drying. Even so, when released into air, the bacterium will survive for up to one hour, although this could vary depending on conditions.

Pneumonic plague is one of several forms of plague. Depending on circumstances, these forms may occur separately or in combination:

• Pneumonic plague occurs when Y. pestis infects the lungs. This type of plague can spread from person to person through the air. Transmission can take place if someone breathes in aerosolized bacteria, which could happen in a bioterrorist attack. Pneumonic plague is also spread by breathing in Y. pestis suspended in respiratory droplets from a person (or animal) with pneumonic plague. Becoming infected in this way usually requires direct and close contact with the ill person or animal. Pneumonic plague may also occur if a person with bubonic or septicemic plague is untreated and the bacteria spread to the lungs.
• Bubonic plague is the most common form of plague. This occurs when an infected flea bites a person or when materials contaminated with Y. pestis enter through a break in a person’s skin. Patients develop swollen, tender lymph glands (called buboes) and fever, headache, chills, and weakness. Bubonic plague does not spread from person to person.
• Septicemic plague occurs when plague bacteria multiply in the blood. It can be a complication of pneumonic or bubonic plague or it can occur by itself. When it occurs alone, it is caused in the same ways as bubonic plague; however, buboes do not develop. Patients have fever, chills, prostration, abdominal pain, shock, and bleeding into skin and other organs. Septicemic plague does not spread from person to person.

Symptoms and Treatment

With pneumonic plague, the first signs of illness are fever, headache, weakness, and rapidly developing pneumonia with shortness of breath, chest pain, cough, and sometimes bloody or watery sputum. The pneumonia progresses for 2 to 4 days and may cause respiratory failure and shock. Without early treatment, patients may die.

Early treatment of pneumonic plague is essential. To reduce the chance of death, antibiotics must be given within 24 hours of first symptoms. Streptomycin, gentamicin, the tetracyclines, and chloramphenicol are all effective against pneumonic plague.

Antibiotic treatment for 7 days will protect people who have had direct, close contact with infected patients. Wearing a close-fitting surgical mask also protects against infection.

A plague vaccine is not currently available for use in the United States.

The plague bacteria can be transmitted to humans in the following ways:

Flea bites. Plague bacteria are most often transmitted by the bite of an infected flea. During plague epizootics, many rodents die, causing hungry fleas to seek other sources of blood. People and animals that visit places where rodents have recently died from plague are at risk of being infected from flea bites. Dogs and cats may also bring plague-infected fleas into the home. Flea bite exposure may result in primary bubonic plague or septicemic plague.

Contact with contaminated fluid or tissue. Humans can become infected when handling tissue or body fluids of a plague-infected animal. For example, a hunter skinning a rabbit or other infected animal without using proper precautions could become infected with plague bacteria. This form of exposure most commonly results in bubonic plague or septicemic plague.

Infectious droplets. 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. Typically this requires direct and close contact with the person with pneumonic plague. Transmission of these droplets is the only way that plague can spread between people. This type of spread has not been documented in the United States since 1924, but still occurs with some frequency in developing countries. Cats are particularly susceptible to plague, and can be infected by eating infected rodents. Sick cats pose a risk of transmitting infectious plague droplets to their owners or to veterinarians. Several cases of human plague have occurred in the United States in recent decades as a result of contact with infected cats.

Plague symptoms depend on how the patient was exposed to the plague bacteria. Plague can take different clinical forms, but the most common are bubonic, pneumonic, and septicemic.

Bubonic plague: The incubation period of bubonic plague is usually 2 to 8 days. Patients develop fever, headache, chills, and weakness and one or more swollen, painful lymph nodes (called buboes). This form usually results from the bite of an infected flea. The bacteria multiply in a lymph node near where the bacteria entered the human body. If the patient is not treated with the appropriate antibiotics, the bacteria can spread to other parts of the body.

Septicemic plague: The incubation period of septicemic plague is poorly defined but likely occurs within days of exposure. 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 symptom 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: The incubation period of pneumonic plague is usually just 1 to 3 days. 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 may develop from untreated bubonic or septicemic plague after the bacteria spread 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).

Plague is a serious illness. If you are experiencing symptoms like those listed here, seek immediate medical attention. Prompt treatment with the correct medications is critical to prevent complications or death.

Plague is a plausible diagnosis for people who are sick and live in, or have recently traveled to, the western United States or any other plague-endemic area. The most common sign of bubonic plague is the rapid development of a swollen and painful lymph gland called a bubo. A known flea bite or the presence of a bubo may help a doctor to consider plague as a cause of the illness.

In many cases, particularly in septicemic and pneumonic plague, there are no obvious signs that indicate plague. Diagnosis is made by taking samples from the patient, especially blood or part of a swollen lymph gland, and submitting them for laboratory testing. Once plague has been identified as a possible cause of the illness, appropriate treatment should begin immediately.

Plague is a very serious illness, but is treatable with commonly available antibiotics. The earlier a patient seeks medical care and receives treatment that is appropriate for plague, the better their chances are of a full recovery.

People in close contact with very sick pneumonic plague patients may be evaluated and possibly placed under observation. Preventive antibiotic therapy may also be given, depending on the type and timing of personal contact.

If you live or have recently traveled to the western U.S. or any other plague endemic area and have symptoms suggestive of plague, seek health care immediately.

1. Reduce rodent habitat around your home, work place, and recreational areas. Remove brush, rock piles, junk, cluttered firewood, and possible rodent food supplies, such as pet and wild animal food. Make your home and outbuildings rodent-proof.
2. Wear gloves if you are handling or skinning potentially infected animals to prevent contact between your skin and the plague bacteria. Contact your local health department if you have questions about disposal of dead animals.
3. Use repellent if you think you could be exposed to rodent fleas during activities such as camping, hiking, or working outdoors. Products containing DEET can be applied to the skin as well as clothing and products containing permethrin can be applied to clothing (always follow instructions on the label).
4. Keep fleas off of your pets by applying flea control products. Animals that roam freely are more likely to come in contact with plague infected animals or fleas and could bring them into homes. If your pet becomes sick, seek care from a veterinarian as soon as possible.
5. Do not allow dogs or cats that roam free in endemic areas to sleep on your bed.

Note: A plague vaccine is no longer available in the United States. New plague vaccines are in development but are not expected to be commercially available in the immediate future.

What is plague?

Plague is a disease caused by bacteria. People usually get plague after being bitten by infected fleas or by handling an infected animal. Rarely, a person with plague can spread the disease to others.

There are three types of plague: bubonic, septicemic, and pneumonic.

Who is at risk?

Plague is found in countries around the world, including certain regions of Africa, central Asia, the Indian subcontinent, the northern part of South America, and parts of the southwestern United States. Rare, urban outbreaks of plague have been reported in Madagascar.

Though plague is found in countries around the world, the risk to travelers is low. However, camping, hiking, or working outdoors can increase your risk.

What can travelers do to prevent plague?

There is no vaccine or medicine that prevents plague available in the United States. Travelers can protect themselves from plague by taking the following precautions.

Use insect repellent

• Use Environmental Protection Agency (EPA)-registered insect repellents with one of the active ingredients below. When used as directed, EPA-registered insect repellents are proven safe and effective, even for pregnant and breastfeeding women.
  • DEET
  • Picaridin (known as KBR 3023 and icaridin outside the US)
  • IR3535
  • Oil of lemon eucalyptus (OLE)
  • Para-menthane-diol (PMD)
  • 2-undecanone
• Find the right insect repellent for you by using EPA’s search tool.

Treat clothing and gear with permethrin

• Use 0.5% permethrin to treat clothing and gear (such as boots, pants, socks, and tents) or buy permethrin-treated clothing and gear.
  • Permethrin is an insecticide that kills or repels mosquitoes.
  • Permethrin-treated clothing provides protection after multiple washings..
  • Read product information to find out how long the protection will last.
• If treating items yourself, follow the product instructions.
• Do not use permethrin products directly on skin.

Do not touch sick or dead animals. Live fleas can jump from a dead animal onto a person.

• If you must handle a sick or dead animal, wear gloves, cover your skin, and use additional personal protective equipment

Additional steps to take if visiting or living in an area with plague

• Store all food in rodent-proof containers.
• Keep rodents out of the house.
• Protect yourself and pets
  • Do not allow pets to sleep with you.
  • Do not allow pets to roam free in known plague areas.
  • Protect your pet from fleas. Talk to a veterinarian.

If you traveled and feel sick, particularly if you have a fever, talk to a healthcare provider, and tell them about your travel. Avoid contact with other people while you are sick.

Plague is an infectious disease that can cause serious illness in people living in rural areas of the western United States and parts of Africa, Asia, and South America. It is caused by the bacteria Yersinia pestis, and people usually get plague after being bitten by an infected flea or handling an infected animal. It is possible that Y. pestis could be released intentionally in a biological attack to sicken people. Public health authorities, including CDC, have prepared for the possible but unlikely event that Y. pestis could be used as a biological weapon.

Although the threat of a bioterrorist attack using Yersinia pestis, the germ that causes plague, is unlikely, there are examples throughout history of Y. pestis being used as a bioweapon. If such an emergency were to occur in the United States, CDC and other federal agencies would work closely with state and local partners to coordinate a response.

Why is plague a concern?

There is concern about plague and bioterrorism because without prompt treatment, the disease can cause serious illness or death. Y. pestis has been classified in the highest risk category of agents with potential to be used as a biological weapon. As a Tier 1 agent, Y. pestis, along with other select agents and toxins, presents the greatest risk of deliberate misuse and harm. This type of bacteria exists in nature in many parts of the world and could be grown in a laboratory in a form that could be released in the air. Even so, manufacturing an effective bioweapon using Y. pestis would require advanced knowledge and technology.

If the bacteria that cause plague were used in a bioterrorist attack, people exposed to the bacteria would be at risk of getting sick but could take antibiotics to prevent illness. There is no vaccine currently available in the United States to prevent plague. New plague vaccines are in development but are not expected to be commercially available in the immediate future.

What might a bioterrorist attack with plague look like?

An outbreak of plague following the use of Y. pestis as a bioweapon could be very different from a naturally occurring plague outbreak. Y. pestis would most likely be used in an aerosol attack, with tiny particles or droplets containing Y. pestis being released in the air (for example, dusts, mists, or fumes). People who inhale Y. pestis could then develop pneumonic plague (infection of the lungs). Pneumonic plague is the most serious form of the disease and the only form that can spread from person to person. As a result of a bioterrorist attack, sick people would likely be seen outside the areas where plague naturally occurs due to the potential for release in other areas and person-to-person spread between close contacts. The number of people infected in a bioterrorism outbreak would depend on many factors, including the area of distribution, quantity of Y. pestis distributed, and environmental conditions like wind and rain.

Though we do not know if or when an attack with Yersinia pestis might take place, CDC and other federal agencies are prepared to respond in case such an emergency occurs.

What you can do to prepare

Preparing for a bioterrorist attack is different from preparing for other emergencies. There are things you can do now to make sure you have the knowledge and tools to protect yourself and your family should an attack occur.

Know how you would stay informed

If a bioterrorist attack using Y. pestis were to occur, CDC and other federal agencies would communicate with you through television, radio, the internet, and other channels. You would be able to get information on CDC’s website or social media pages. CDC would coordinate with state and local public health departments, law enforcement, and other federal agencies to let you know the things you can do to protect yourself and your family. You can also talk to your primary healthcare provider.

In any public health emergency, it’s important for you to look for information early and often. As CDC and other public health authorities learn more information about the situation, they will communicate it to you.

Know your medical history

If an attack with Y. pestis were to occur, public health authorities would distribute antibiotics and medical supplies to communities to help prevent infection. Antibiotic prevention (prophylaxis) and treatments for plague are generally safe and effective but may cause some side effects. Before taking antibiotics or giving antibiotics to others in your household, you would need to tell treating healthcare providers about the medical history of anyone in your family who is to receive antibiotics, including:

• Their medical conditions
• Any medications they are currently taking
• Allergies to drugs or medications

In a plague emergency, many lives can be saved with prompt prevention and treatment. It would be very important to start taking antibiotics as soon as you get them and take them as directed.

Know the symptoms of plague

During a plague emergency, you should be able to recognize the symptoms of plague, especially pneumonic plague, and be prepared to get medical care if symptoms develop.

If a bioterrorist attack using Yersinia pestis were to occur, infected people would likely show signs of illness within 1-3 days. Early in the outbreak, when the first wave of people get sick, healthcare providers might not recognize signs of plague in their patients, especially in areas of the country where plague does not usually occur. Public health authorities would likely recognize a bioterrorist attack as more people became sick and when laboratory test results became available.

How would CDC respond to a plague emergency?

Several cases of pneumonic plague in one area would be a public health emergency, prompting CDC to respond immediately. Additionally, any case of plague outside areas where plague naturally occurs and in people without history of travel would cause alarm. In a plague emergency, CDC and other federal agencies would work closely with state and local partners to coordinate a response.

CDC and other federal agencies would respond by:

• Coordinating activities through the Emergency Operations Center.
• Communicating information about plague to the public.
• Providing guidance to healthcare providers, hospitals, health departments, and others on how to respond.
• Working with the Laboratory Response Network (LRN) to test samples from sick people.
• Sending out field staff to interview people who are sick with plague and those who were in close contact with them.
• Shipping out antibiotic treatments and other medical supplies from the Strategic National Stockpile (SNS) to states and other locations where needed.

What is plague?

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.

What are the different forms of plague?

There are three forms of plague:

• Bubonic plague: Patients develop fever, headache, chills, and weakness and one or more swollen, painful lymph nodes (called buboes). This form is usually the result of an infected flea bite. The bacteria multiply in a lymph node near 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.

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.

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.

What is the incubation period for plague?

A person usually becomes ill with bubonic plague 2 to 8 days after being infected. The incubation period of septicemic plague is poorly defined but likely occurs within days of exposure. A person exposed to Yersinia pestis through the air would usually become ill in just 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.

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.

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 Organization (WHO), though the true number is likely much higher.

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.

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.

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.

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.

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.

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.