<P> The importance of hygiene was recognised only in the nineteenth century; until then it was common that the streets were filthy, with live animals of all sorts around and human parasites abounding . A transmissible disease will spread easily in such conditions . One development as a result of the Black Death was the establishment of the idea of quarantine in Dubrovnik in 1377 after continuing outbreaks . </P> <P> The dominant explanation for the Black Death is the plague theory, which attributes the outbreak to Yersinia pestis, also responsible for an epidemic that began in southern China in 1865, eventually spreading to India . The investigation of the pathogen that caused the 19th - century plague was begun by teams of scientists who visited Hong Kong in 1894, among whom was the French - Swiss bacteriologist Alexandre Yersin, after whom the pathogen was named Yersinia pestis . The mechanism by which Y. pestis was usually transmitted was established in 1898 by Paul - Louis Simond and was found to involve the bites of fleas whose midguts had become obstructed by replicating Y. pestis several days after feeding on an infected host . This blockage results in starvation and aggressive feeding behaviour by the fleas, which repeatedly attempt to clear their blockage by regurgitation, resulting in thousands of plague bacteria being flushed into the feeding site, infecting the host . The bubonic plague mechanism was also dependent on two populations of rodents: one resistant to the disease, which act as hosts, keeping the disease endemic, and a second that lack resistance . When the second population dies, the fleas move on to other hosts, including people, thus creating a human epidemic . </P> <P> The historian Francis Aidan Gasquet wrote about the Great Pestilence in 1893 and suggested that "it would appear to be some form of the ordinary Eastern or bubonic plague". He was able to adopt the epidemiology of the bubonic plague for the Black Death for the second edition in 1908, implicating rats and fleas in the process, and his interpretation was widely accepted for other ancient and medieval epidemics, such as the Justinian plague that was prevalent in the Eastern Roman Empire from 541 to 700 CE . </P> <P> An estimate of the mortality rate for the modern bubonic plague, following the introduction of antibiotics, is 11%, although it may be higher in underdeveloped regions . Symptoms of the disease include fever of 38--41 ° C (100--106 ° F), headaches, painful aching joints, nausea and vomiting, and a general feeling of malaise . Left untreated, of those that contract the bubonic plague, 80 per cent die within eight days . Pneumonic plague has a mortality rate of 90 to 95 per cent . Symptoms include fever, cough, and blood - tinged sputum . As the disease progresses, sputum becomes free - flowing and bright red . Septicemic plague is the least common of the three forms, with a mortality rate near 100% . Symptoms are high fevers and purple skin patches (purpura due to disseminated intravascular coagulation). In cases of pneumonic and particularly septicemic plague, the progress of the disease is so rapid that there would often be no time for the development of the enlarged lymph nodes that were noted as buboes . </P>

Where did the black death kill the most