Causative agent of plagueresistant to low temperatures, well preserved in sputum, but at 55 ° C dies for 10-15 minutes, and when boiled - almost immediately. It enters the body through the skin (with the bite of a flea, as a rule, Xenopsylla cheopis), the mucous membranes of the respiratory tract, the digestive tract, the conjunctiva.
According to the main carrier, natural plague foci are divided into ground squirrel, groundhog, sandworm, field-dipper, and pinworm. In addition to wild rodents, the epizootic process sometimes includes the so-called synanthropic rodents (in particular, rats and mouse-like), as well as some wild animals (hares, foxes), which are the object of hunting. From domestic animals camels suffer from plague.
According to some modern data, Y.pestis is an obligate parasite living in soil single-celled organisms (amoebae), that is, the natural reservoir of the plague pathogen is a collection of unicellular organisms - the biological hosts of Y.pestis. Infected rodents should not be viewed as a natural reservoir of the plague, but as an indicator of a shift in the ecology of unicellular organisms - the natural hosts of Y.pestis.Those territories in which the plague pathogen is supported by epizooties among rodents should be considered as the tip of the plague iceberg. This allows us to make the assumption that Y.pestis in the human body and rodents shows virulence in the form of a local phenomenon - the ability to proliferate in white blood cells. At the same time, Y.pestis uses the same specialization mechanisms that allow it to be maintained among soil single-celled organisms - historical predecessors of phagocytes. Since the life or death of an accidentally infected warm-blooded organism does not mean anything to maintain such a parasite in nature, its virulence is not limited by the need to save the lives of its victims. 
In a natural outbreak, infection usually occurs through a flea bite previously fed on a sick rodent, the probability of infection increases significantly when the synanthropic rodents are included in the epizootic. Infection occurs also when hunting for rodents and their further processing. Mass diseases of people occur during trimming of a sick camel, skinning, cutting, processing.An infected person, depending on the form of the disease, in turn, can be the transmitter of the plague by airborne droplets or through the bite of certain types of fleas.
Fleas are a specific carrier of the plague pathogen. This is due to the characteristics of the device of the digestive system of fleas: just before the stomach, the esophagus of the flea forms a thickening - goiter. When an infected animal (rat) bites, the plague bacteria accumulates in the flea's goiter and begins to multiply rapidly, completely clogging it. The blood cannot get into the stomach, so this flea is permanently tormented by the feeling of hunger. She goes from owner to owner in the hope of receiving her portion of blood and manages to infect a sufficiently large number of people before she dies (such fleas do not live for more than ten days).
When a flea is infected by a plague bacteria, a person may have a papule or pustule at the site of the bite, filled with hemorrhagic contents (skin form). Then the process spreads through the lymphatic vessels without the manifestation of lymphangitis. The reproduction of bacteria in the macrophages of the lymph nodes leads to their sharp increase, merger and the formation of conglomerate (bubonic form).Further generalization of the infection, which is not strictly obligatory, especially in the conditions of modern antibacterial therapy, can lead to the development of a septic form, accompanied by the defeat of almost all internal organs. However, from an epidemiological point of view, the “screening out” of infection in the lung tissue with the development of the pulmonary form of the disease plays an important role. From the moment of the development of plague pneumonia, a sick person becomes a source of infection, but at the same time a pulmonary form of the disease is transmitted from person to person - extremely dangerous, with a very rapid course.
The most important role in the diagnosis in modern conditions is played by the epidemiological history. Coming from zones endemic to plague (Vietnam, Burma, Bolivia, Ecuador, Turkmenistan, Karakalpakia, etc.), or from patient's anti-plague stations with the above-described signs of bubonic form or signs of the most severe — with hemorrhages and bloody sputum — pneumonia with marked lymphadenopathy It is a sufficiently serious argument for the physician of the first contact to take all measures to localize the alleged plague and to diagnose it accurately.It is necessary to emphasize in particular that in the conditions of modern medical prophylaxis, the probability of illness of personnel who for some time had been in contact with the plague cough patient is very small. Currently, there have been no cases of primary pneumonic plague (that is, cases of infection from person to person) among medical personnel. Establishing an accurate diagnosis should be carried out using bacteriological studies. The material for them is punctate of the festering lymph node, sputum, patient's blood, discharge of fistula and ulcers.
Laboratory diagnostics is carried out using fluorescent specific antisera, which are stained with smears of detachable ulcers, punctate lymph nodes, and cultures obtained on blood agar.