Symptoms

Contemporary accounts of the plague are often varied or imprecise. The most commonly noted symptom was the appearance of buboes (or gavocciolos) in the groin, the neck and armpits, which oozed pus and bled when opened. Boccaccio's description is graphic:

"In men and women alike it first betrayed itself by the emergence of certain tumours in the groin or armpits, some of which grew as large as a common apple, others as an egg...From the two said parts of the body this deadly gavocciolo soon began to propagate and spread itself in all directions indifferently; after which the form of the malady began to change, black spots or livid making their appearance in many cases on the arm or the thigh or elsewhere, now few and large, now minute and numerous. As the gavocciolo had been and still was an infallible token of approaching death, such also were these spots on whomsoever they showed themselves."

Ziegler comments that the only medical detail that is questionable is the infallibility of approaching death, as if the bubo discharges, recovery is possible.

This was followed by acute fever and vomiting of blood. Most victims died two to seven days after initial infection. David Herlihy identifies another potential sign of the plague: freckle-like spots and rashes which could be the result of flea-bites.

Some accounts, like that of Louis Heyligen, a musician in Avignon who died of the plague in 1348, noted a distinct form of the disease which infected the lungs and led to respiratory problems and which is identified with pneumonic plague.

"It is said that the plague takes three forms. In the first people suffer an infection of the lungs, which leads to breathing difficulties. Whoever has this corruption or contamination to any extent cannot escape but will die within two days. Another form...in which boils erupt under the armpits,...a third form in which people of both sexes are attacked in the groin

Causes

Medical knowledge had stagnated during the Middle Ages. The most authoritative account at the time came from the medical faculty in Paris in a report to the king of France that blamed the heavens, in the form of a conjunction of three planets in 1345 that caused a "great pestilence in the air". This report became the first and most widely circulated of a series of "plague tracts" that sought to give advice to sufferers. That the plague was caused by bad air became the most widely accepted theory. The word plague had no special significance at this time, and only the recurrence of outbreaks during the Middle Ages gave it the name that has become the medical term.

The importance of hygiene was recognized 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.

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 behavior 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.

The historian Francis Aidan Gasquet, who had written about the 'Great Pestilence' in 1893 and suggested that "it would appear to be some form of the ordinary Eastern or bubonic plague" 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 AD.

More recently other forms of plague have been implicated. The modern bubonic plague has a mortality rate of 30 to 75 percent and symptoms including fever of 38–41 °C (101–105 °F), headaches, painful, aching joints, nausea and vomiting, and a general feeling of malaise. If untreated, of those that contract the bubonic plague, 80 percent die within eight days. Pneumonic plague has mortality rate of 90 to 95 percent. 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 close to 100 percent. 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.

"Many modern scholars accept that the lethality of the Black Death stemmed from the combination of bubonic and pneumonic plague with other diseases and warn that every historical mention of 'pest' was not necessarily bubonic plague...In her study of 15th-century outbreaks, Ann Carmichael states that worms, the pox, fevers and dysentery clearly accompanied bubonic plague