Passage 1 Plague Plague is primarily a disease of densely populated zones, households above all else, and its perpetuation and reinforcement involve complex relationship among wild rodents, bacteria, insect vector, and humans. Occasional plague outbreaks can occur almost anywhere, but for the disease to become rooted an environment must exist which can support a high density of rodent hosts living in proximity with human beings. Endemic plague among rodents remote from human habitation poses a danger only to humans who invade those zones The bacterium responsible for bubonic plague is Yersinia(formerly Pasteurella pestis, one of the most pervasive, persistent, and dangerous life forms in the world. It lives in the bloodstream of dozens of varieties of rodents(rats, ground squirrel marmots,etc. ), but it is most commonly found among rats. The simple presence of Y. Pestis in the rodent population has not always meant an immediate threat to humankind. Once the disease becomes epizootic among rodents in proximity to people, however, epidemic outbreaks have followed with consequences equally disastrous to humans and to the rodent hosts Why an epizootic starts is not clear. The bacterium begins to multiply. As it does SO, it poisons its hosts blood, and the rodent dies. This is why a"rat-fall', the sudden appearance of dead or dying rats has come to be regarded as the precursor of a human epidemic. The most common vector for transferring Y. Pestis from one body to another is the rat flea, Xenopsylla cheopis, though other flea types are almost as efficient. Among the some 200 known fleas which transfer plague, one of the least efficient is the human body flea(Pulex irritans), and it therefore seldom happens that flea-borne plague will travel from one person to another Feeding fleas transfer Y. Pestis from rat to rat or from rat to human. The flea lives in the rat s fur and feeds on blood through the skin when the flea is on an infected rat it draws Y. Pestis into its own stomach with the blood it ingests. In turn, the plague bacterium multiplies in the fleas stomach, eventually plugging it with live bacteria The flea, which can no longer swallow fresh blood, regurgitates live bacteria when next it tries to feed, and those bacteria pass through the break in the skins surface The flea defecates as it attempts to feed, and this provides another source for infective material When the host rat dies and its body cools, the blocked or engorged flea moves or to the next living creature available. If that creature is humane the flea will bite in an effort to feed, thereby introducing Y. Pestis into the human bloodstream. A bubonic infection follows. Rats are the essential plague incubators. Even humans dying of plague have too little of the bacterial agent in their bloodstream to continue the nfective chain, and modern physicians believe that plague victims can be nursed in an pen ward without danger. The exceptions are patients with pneumonic plague, where the bacteria have entered the lungs(rather than remaining in the bloodstream) to b expelled in droplets with every breath or cough. Pneumonic plague is highly contagious, and it is almost al ways fatalPassage 1 Plague Plague is primarily a disease of densely populated zones, households above all else, and its perpetuation and reinforcement involve complex relationship among wild rodents, bacteria, insect vector, and humans. Occasional plague outbreaks can occur almost anywhere, but for the disease to become rooted, an environment must exist which can support a high density of rodent hosts living in proximity with human beings. Endemic plague among rodents remote from human habitation poses a danger only to humans who invade those zones. The bacterium responsible for bubonic plague is Yersinia (formerly Pasteurella) pestis, one of the most pervasive, persistent, and dangerous life forms in the world. It lives in the bloodstream of dozens of varieties of rodents (rats, ground squirrels, marmots, etc.), but it is most commonly found among rats. The simple presence of Y. Pestis in the rodent population has not always meant an immediate threat to humankind. Once the disease becomes epizootic among rodents in proximity to people, however, epidemic outbreaks have followed with consequences equally disastrous to humans and to the rodent hosts. Why an epizootic starts is not clear. The bacterium begins to multiply. As it does so, it poisons its host’s blood, and the rodent dies. This is why a “rat-fall”, the sudden appearance of dead or dying rats has come to be regarded as the precursor of a human epidemic. The most common vector for transferring Y. Pestis from one body to another is the rat flea, Xenopsylla cheopis, though other flea types are almost as efficient. Among the some 200 known fleas which transfer plague, one of the least efficient is the human body flea (Pulex irritans), and it therefore seldom happens that flea-borne plague will travel from one person to another. Feeding fleas transfer Y. Pestis from rat to rat or from rat to human. The flea lives in the rat’s fur and feeds on blood through the skin. When the flea is on an infected rat, it draws Y. Pestis into its own stomach with the blood it ingests. In turn, the plague bacterium multiplies in the flea’s stomach, eventually plugging it with live bacteria. The flea, which can no longer swallow fresh blood, regurgitates live bacteria when next it tries to feed, and those bacteria pass through the break in the skin’s surface. The flea defecates as it attempts to feed, and this provides another source for infective material. When the host rat dies and its body cools, the blocked or engorged flea moves on to the next living creature available. If that creature is humane, the flea will bite in an effort to feed, thereby introducing Y. Pestis into the human bloodstream. A bubonic infection follows. Rats are the essential plague incubators. Even humans dying of plague have too little of the bacterial agent in their bloodstream to continue the infective chain, and modern physicians believe that plague victims can be nursed in an open ward without danger. The exceptions are patients with pneumonic plague, where the bacteria have entered the lungs (rather than remaining in the bloodstream) to be expelled in droplets with every breath or cough. Pneumonic plague is highly contagious, and it is almost always fatal