Plague disease

Plague is a severe infectious disease caused by the bacterium Yersinia pestis, a member of the Enterobacteriaceae family. It is a zoonotic illness that historically produced some of the most devastating pandemics in human history, including the fourteenth-century Black Death, which killed tens of millions of people in Europe. Although modern antibiotics have greatly reduced mortality, plague remains a public health concern in several parts of the world, with hundreds of cases reported annually.
The disease manifests in multiple clinical forms, each affecting different systems of the body and associated with distinct symptoms. Transmission usually occurs via infected fleas or host animals, but human-to-human spread is possible, especially in pneumonic plague. Early diagnosis and antibiotic therapy are crucial for reducing the high mortality rate associated with untreated infections.

Clinical forms and symptoms

Plague presents in several clinical forms, the most common being bubonic plague, followed by septicemic plague and pneumonic plague. Less common manifestations include plague meningitis, pharyngitis and ocular infections. General symptoms across most forms include fever, chills, headaches, nausea and profound weakness.
• Bubonic plagueThis form occurs when Y. pestis is introduced into body tissues, typically through the bite of an infected flea. The bacterium multiplies locally and then spreads via the lymphatic system, causing inflammation and swelling of lymph nodes. These swollen, painful nodes—called buboes—are a hallmark of the disease. Autopsies often reveal haemorrhage and necrosis within the buboes. If untreated, the infection may spread into the bloodstream, causing secondary septicemia, or seed the lungs, leading to secondary pneumonic plague.
• Septicemic plagueWhen Y. pestis enters the bloodstream directly or following bubonic infection, rapid bacterial multiplication triggers disseminated intravascular coagulation, forming microscopic clots throughout the body. These cause tissue ischaemia and necrosis and consume clotting factors, leading to widespread bleeding. Symptoms include abdominal pain, vomiting or coughing up blood, patchy red or black discolouration of the skin, and shock. Without treatment, septicemic plague is almost always fatal; antibiotic therapy reduces mortality significantly.
• Pneumonic plagueThis form arises when the lungs are infected either by inhaling infectious droplets or by blood-borne spread. Pneumonic plague is the most contagious form, as coughing generates infectious respiratory droplets. The incubation period is short—hours to a few days—and symptoms such as fever, weakness, chest pain, and haemoptysis progress rapidly. Without prompt antibiotic treatment, mortality approaches 100 per cent, often within a few days.

Transmission and natural reservoirs

Yersinia pestis circulates naturally in rodent populations and their associated flea vectors. The tropical and subtropical regions, as well as warm temperate zones between roughly 55°N and 40°S, contain numerous natural foci where the bacterium persists. Fleas of the species Xenopsylla cheopis are the primary vectors in many plague-affected regions of Africa, Asia, and South America.
Transmission can occur through several pathways:

• Flea bites following feeding on an infected rodent host
• Contact with tissues or fluids from infected animals
• Inhalation of infectious respiratory droplets in pneumonic plague
• Less commonly, contact with contaminated surfaces or food

Contrary to common misconception, rats do not directly infect humans; instead, infected fleas serve as the true vectors. Within the flea, the bacteria multiply and form a plug in the digestive tract. This prevents the flea from feeding properly and causes it to regurgitate infected material into bite wounds. Outbreaks in human populations often follow die-offs or surges in rodent numbers, increasing opportunities for flea transmission.
Historical analysis, including studies of the 1665 Eyam outbreak in England, indicates that human-to-human transmission played a larger role in some epidemics than previously thought, especially within households during periods of close contact.

Diagnosis

Plague cannot be reliably diagnosed based on symptoms alone, as early signs resemble many other febrile illnesses. Laboratory testing is therefore essential. Diagnostic confirmation involves identifying Y. pestis in samples obtained from blood, sputum, or bubo aspirates. Standard methods include microscopy and bacterial culture.
Additional diagnostic tools include:

• Polymerase chain reaction (PCR) tests that detect specific bacterial genes and can identify both live and dead organisms
• Serological tests that measure rising antibody titres between acute and convalescent blood samples
• Rapid diagnostic tests such as F1 antigen detection for use in suspected pneumonic or bubonic cases

Antibiotic administration prior to sampling or poor sample handling may lead to false negatives in culture, although PCR remains effective under such circumstances.

Treatment and prevention

Plague is treatable with antibiotics, especially when administered early. Agents such as gentamicin and fluoroquinolones are commonly used. With prompt treatment, mortality falls to about 10 per cent, but without therapy, it may reach 70 per cent or higher.
Preventive measures include:

• Early identification and isolation of pneumonic plague cases
• Prophylactic antibiotics for individuals exposed to pneumonic cases
• Vector control programmes targeting rodent populations and fleas
• Use of plague vaccines for high-risk groups in endemic regions

Supporting care such as fluid management and respiratory support improves outcomes in severe cases.

Epidemiology and historical impact

Plague remains endemic in several countries. Regions with regular reporting include Madagascar, the Democratic Republic of the Congo, and Peru. In the United States, sporadic cases occur in rural western states where Y. pestis circulates among wild rodents.
Historically, plague has shaped demographic and social developments. The Black Death of the fourteenth century resulted in tens of millions of deaths across Europe and profoundly altered economic and cultural structures. Earlier and later pandemics also occurred, affecting Asia, Africa, and the Mediterranean world over many centuries.

Public health significance

Although much better controlled today, plague persists in natural reservoirs, making eradication unlikely. Continued surveillance, rapid diagnostic capacity, vector management and public health preparedness remain essential to limiting outbreaks. Modern research continues to refine diagnostic tools, improve treatments and understand the ecology of Y. pestis to reduce the global burden of this enduring infectious disease.