Plague is an acute, contagious, febrile illness transmitted to humans by the bite of an infected rat flea. Human-to-human transmission is rare except during epidemics of pneumonic plague. The disease is caused by the plague bacillus, rod-shaped bacteria referred to as Yersinia pestis. Yersinia is named in honor of Alexander Yersin, who successfully isolated the bacteria in 1894 during the pandemic that began in China in the 1860s.
Plague is worldwide in distribution, with most of the human cases reported from developing countries.
Three studies have shown that this bacterium emerged from the gut pathogen Yersinia pseudotuberculosis shortly after the first epidemic.  Three biovars (with minor genetic variations) have been identified within the Y pestis clone—Antiqua, Medievalis, and Orientalis.  One theory is that these biovars emerged before any of the plague epidemics. In fact, as reported by Drancourt et al (2004), genotyping performed on bacteria derived from the remains of plague victims of the first two epidemics revealed sequences similar to that of Orientalis. 
The virulence of this bacterium results from the 32 Y pestis chromosomal genes and two Y pestis –specific plasmids, constituting the only new genetic material acquired since its evolution from its predecessor.  These acquired genetic changes have allowed the pathogen to colonize fleas and to use them as vectors for transmission. 
Plague is a zoonotic disease that primarily affects rodents; humans are incidental hosts. Dog-to-human transmission was reported in a 2014 outbreak in Colorado.  Survival of the bacillus in nature depends on flea-rodent interaction, and human infection does not contribute to the bacteria's persistence in nature. Of the 1500 flea species identified, only 30 of them have been shown to act as vectors of plague.  The most prominent of these vectors is Xenopsylla cheopis (oriental rat flea); however, Oropsylla montana has been incriminated as the primary vector for this disease in North America. 
Host fatality has been known to be the harbinger of an epidemic.  Whether susceptibility and fatality are related is unknown. However, ground squirrels and prairie dogs have been known to be highly susceptible to plague, whereas others have been known to be either moderately susceptible or absolutely resistant to infection.
Plague was first described in the Old Testament and has persisted into the modern era. Plague has caused large-scale epidemics, thereby changing the course of history in many nations. The first pandemic was believed to have started in Africa and killed 100 million people over a span of 60 years. In the Middle Ages, plague killed approximately one fourth of Europe's population. The pandemic that began in China in the 1860s spread to Hong Kong in the 1890s and was subsequently spread by rats transported on ships to Africa, Asia, California, and port cities of South America. In the early twentieth century, plague epidemics accounted for about 10 million deaths in India. As reported in National Geographic, mass graves of plague victims were recently discovered in an area of Venice called "Quarantine Island."
Y pestis is a nonmotile, pleomorphic, gram-negative coccobacillus that is nonsporulating. The bacteria elaborate a lipopolysaccharide endotoxin, coagulase, and a fibrinolysin, which are the principal factors in the pathogenesis of plague. The pathophysiology of plague basically involves two phases—a cycle within the fleas and a cycle within humans.
The key to the organism’s virulence is the phenomenon of "blockage," which aids the transmission of bacteria by fleas. After ingestion of infected blood, the bacteria survive in the midgut of the flea owing to a plasmid-encoded phospholipase D that protects them from digestive juices.  The bacteria multiply uninhibited in the midgut to form a mass that extends from the stomach proximally into the esophagus through a sphincterlike structure with sharp teeth called the proventriculus.
It has been shown that this property requires the presence of hemin-producing genes, which are needed for the formation of a biofilm that permits colonization of the proventriculus.  In fact, as described by Jarrett et al (2004), this mutation in hemin genes allows colonization in the midgut without extension to the proventriculus. Consequently, the "blockage phenomenon" does not occur, thereby leading to failure of transmission.  This blockage causes the flea to die of starvation and dehydration. 
As a desperate measure, the flea then repeatedly tries to obtain a meal by biting a host, managing only to regurgitate the infected mass into host's bloodstream. However, the concept that the flea must be engorged before becoming infectious loses support when trying to explain the rapid rate of spread of disease during a plague epidemic. Studies of vectors such as O montana clearly indicate the redundancy of the aforementioned hypothesis, since this vector does not die of blockage and remains infectious for a long period, unlike its counterpart. 
Once the flea bites a susceptible host, the bacilli migrate to the regional lymph nodes, are phagocytosed by polymorphonuclear and mononuclear phagocytes, and multiply intracellularly. Survival and replication within macrophages is probably of greatest importance in early stages of the disease.  Involved lymph nodes show dense concentrations of plague bacilli, destruction of the normal architecture, and medullary necrosis. With subsequent lysis of the phagocytes, bacteremia can occur and may lead to invasion of distant organs in the absence of specific therapy.
The following are the modes of plague transmission in humans: 
- Bites by fleas
- Exposure to humans with pneumonic plague
- Handling of infected carcasses
- Scratches or bites from infected domestic cats
- Exposure to aerosols containing plague-causing bacilli
Another potential cause of plague transmission in humans is contact with an infected dog. In 2014, the Colorado Department of Public Health and Environment (CDPHE) laboratory isolated Y pestis in a blood specimen from a hospitalized man with pneumonia. Further investigation found that the man’s dog had recently died with hemoptysis and that 3 other persons who came into contact with the dog had respiratory symptoms and fever. Specimens from the dog and the other three persons showed evidence of acute Y pestis infection. One of the transmissions may have been human to human, which would be the first such reported US case since 1924. 
A few natural plague foci are located in the western United States. From the states of Arizona, California, Colorado, New Mexico, and Utah, 49 cases of plague and 3 attributed deaths were reported from 1994-1999.  In 2006, 13 plague cases were reported among residents of New Mexico, Colorado, California, and Texas, two of which resulted in death. 
On average, 10-15 cases of human plague are reported annually in the United States. Over time, human cases of plague have moved from crowded cities to the rural West. This has paralleled the observed patterns of introduction of exotic plants and animals.  The rate of plague in the United States is low, since most of the endemic areas are rural and largely uninhabited, thereby limiting human exposure. In recent years, and with the potential threat for bioterrorism, the Centers for Disease Control and Prevention (CDC) has specified Y pestis as a Category A bioterrorism agent.
Animal reservoirs in America mostly include squirrels, rabbits, and prairie dogs. However, there has been an established role of domestic cats in the transmission of plague since the late 1970s. Since 1977-1998, 23 cases of human plague associated with cats have been reported from the western states, representing 8% of all reported plague cases during that time.  In this scenario, transmission via inhalation was more common than in any other form of plague.
In a study of cat-related plague, mortality was associated with misdiagnosis or delay in treatment. Of the 23 cases from 1977-1998, 5 of 17 bubonic plague cases resulted in death. 
In 2014, the Colorado Department of Public Health and Environment (CDPHE) laboratory isolated Y pestis in a blood specimen from a hospitalized man with pneumonia. Further investigation found that the man’s dog had recently died with hemoptysis and that 3 other persons who came into contact with the dog had respiratory symptoms and fever. Specimens from the dog and the other three persons showed evidence of acute Y pestis infection. One of the transmissions may have been human to human, which would be the first such reported US case since 1924. 
Most cases of plague reported outside of the United States are from developing countries in Africa and Asia. During 1990-1995, a total of 12,998 cases of plague were reported to the World Health Organization (WHO), particularly from countries such as India, Zaire, Peru, Malawi, and Mozambique. The following countries reported more than 100 cases of plague: China, Congo, India, Madagascar, Mozambique, Myanmar, Peru, Tanzania, Uganda, Vietnam, and Zimbabwe. Several foci are located in the semi-arid regions of northeastern Brazil, and outbreaks have also been reported from Malawi and Zambia. Australia is the only continent that is considered free of plague. The largest enzootic plague area is in North America—the southwestern United States and the Pacific coastal area.
The WHO reports that, in 2003, 9 countries reported a total of 2118 plague cases and 182 deaths, 98.7% and 98.9% of which were reported from Africa, respectively.
The risk of plague-related death depends on the type of plague and whether the infected individual receives appropriate treatment. 
The following are the mortality rates associated with the different types of plague:
- Untreated - 100%
- Treated - 50%
- Untreated - 50%-90%
- Treated - 10%-20%
Septicemic plague - 20%-25%
In the United States, most cases of plague occur in whites. Native Americans living in endemic areas of Arizona, New Mexico, and Utah have a 10-fold greater risk of acquiring the disease than non–Native Americans.
Humans are exposed in the domestic or outdoor environment. Infections in the wild are usually isolated or sporadic, causing infections in Indians, hunters, miners, and tourists in the United States and Brazil.
Plague has no sexual predilection.
Most cases of plague occur in persons younger than 20 years.
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