Plague Glossary

Bubo: A bubo is a swollen lymph node where Yersinia pestis (Y. pestis) is multiplying. It is the most characteristic sign of the plague. It usually occurs in three areas: the groin, the axilla (arm pit), or cervical (tonsils or neck lymph nodes). The location of  a bubo can tell you something about the possible method of transmission. Groin bubos tend to be from flea bites. Axillary buboes can be from animal bites or other wounds, less likely to be flea bites on adults. Cervival bubos can be an indicator of pneumonic transmission by large respiratory droplets indicating close contact. Flea bites on the arm or head can cause axillary or cervical bubos respectively too.

Bubonic plague is an outbreak that is characterized primarily by bubo development. Plague is generally called bubonic unless there is evidence of pneumonic spread. Bubonic plague is spread primarily by flea bites, but can also be spread by animal bites or infectious material contaminating wounds.

Endemic is the amount of a given disease normally found in a specific geographic area. For example, ‘staph infections’ are present in most communities at a certain rate depending on local conditions.

Enzootic is the amount of a given disease that is normally found in an animal population in a specified area. This is the animal equivalent of endemic in humans.

Epidemic is a dramatic increase in a given disease over the normal conditions. The difference between an epidemic and an outbreak is that an epidemic should have evidence of propagation or spread within the population.

Epizootic: an epidemic among local animals. Epizootics usually precede the beginning of a plague outbreak. One of the best indicators of a coming plague problem is an absence of rodents and empty rodent burrows.

Morbidity: rate of illness.

Mortality: rate of fatalities.  Reports of morbidity and mortality are reports of sickness and death.

Outbreak: A sudden incidence of infectious disease in a local area. The difference between an outbreak and an epidemic is that an epidemic propagates or spreads while an outbreak does not necessarily spread. Cases of food poisoning linked to a single event like a potluck or picnic would be considered an outbreak. Outbreaks can develop into epidemics if disease begins to propagate.

Pandemic: The simplest definition of a pandemic is that it is an epidemic on two or more continents. In the case of the Plague of Justinian, it was found in Africa, Europe and Asia. The third plague pandemic began in China, spread through Asia, Africa reaching Madagascar for the first time  and North America.

Pneumonic plague is an outbreak that is characterized primarily by pneumonic transmission, meaning by respiratory droplets. Large droplets settle in the throat and cause bubos in the tonsils or cervical lymph nodes. Small respiratory droplets reach the lungs causing a rapid septicemia (blood borne infection). The rapid development of septicemia is what makes pneumnonic plague so fatal. Most modern cases of pneumonic transmission are small, self-limiting events. Bubonic plague and pneumonic plague can exist within the same epidemic.

Reservoir is the place or organism where an infectious microbe is found in between human outbreaks. For the plague, native rodents around the world are its reservoir. Plague has a talent for adapting to native burrowing rodents around the world. In Europe, rats are usually considered the primary rodent reservoir, but on the Asian steppe the marmot and gerbil are the primary reservoirs and in the United States it is the prairie dog, a variety of squirrels, and native mice. For other microbes, the reservoir could be soil, water or other inanimate objects.

Septicemia is a blood-borne infection. All fatal cases of plague become septicemic before death. This means that all body fluids and body parts can carry disease. Forensic anthropologists are having success finding plague in the pulps of teeth. Septicemia develops immediately in pneumonic plague and can develop after flea bites. Bubos do not always form after flea bites. Blackening of the extremities, called disseminated intravascular coagulation (DIS), can form during septicemia.

Vectors are agents that transmit a microbe from the reservoir to humans. A vector can be an integral part of the microbes lifecycle, such as mosquitoes for malaria.  Insects like mosquitoes, lice, fleas and ticks are common vectors. Mosquitoes are a vector of malaria, and fleas and possibly lice are vectors for plague. Inanimate objects can also be vectors.


Zoonosis is an infectious disease transmitted from animals to humans. These diseases are usually found primarily among animals although some have humans has an integral part of their lifecycle. Plague is a zoonosis.

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