The disease was bubonic plague, present in two forms: one that infected the bloodstream, causing the buboes and internal bleeding, and was spread by contact; and a second, more virulent pneumonic type that infected the lungs and was spread by respiratory infection. The presence of both at once caused the high mortality and speed of contagion. So lethal was the disease that cases were known of persons going to bed well and dying before they woke, of doctors catching the illness at a bedside and dying before the patient. So rapidly did it spread from one to another that to a French physician, Simon de Covino, it seemed as if one sick person "could infect the whole world." The malignity of the pestilence appeared more terrible because its victims knew no prevention and no remedy.
The societal response was amazing. Although some fled to the relative safety of the countryside, and others exploited the occasion for looting, most people saw their duty to provide comfort for the sick or dying. The notion of a 'hospital' in the modern sense hadn't yet developed, so makeshift infirmaries were set up wherever possible. The first line of caregivers were monks and nuns - Benedictines, Dominicans, Augustinians, and Franciscans. But it became clear that more would be needed, especially as caregivers were more likely to be infected, and more likely to die.
True heroism was seen among the ordinary laypeople - farmers and townspeople - who risked their lives, and in some cases almost ensured their deaths by enlisted to care for the ill. This desire to help revealed something profound in Western Civilization: an ethic of service toward one's fellow man. Centuries of culture and tradition had taught this principle, and when European culture faced one of its worst humanitarian disasters, this principle was securely in place among the people.