The notary from Piacenza writes about the attack with human corpses that were thrown by the Mongols lead by Khan Janibeg, cadavers that were bringing the virus to the healthy people in the town. Their response was to pray and hope that God will punish the infidels and keep them safe. Kelly further explains that even if de'Mussins' accounts of the Mongol attack on the Geneoese were fabricated, the epidemic might have had different means of spreading, like the rats who were traveling easily throughout the town and making the contamination a matter of days. Hazard made Caffa to be the town where hell broke loose upon the attack of the Monglos and the Europens who fled in horror brought hell along to Europe. Kelly underlines that what the European historians called the Black Death "has killed an estimated two hundred million people, and no outbreak of plague has claimed as man victims or caused as much anguish and sorrow as the Black Death" (The Great Mortality, p. 11). How this might have happened is what Kelly is attempting to find out, by closely examining the conditions that converged toward the spread of the bacillus in such a short time on such a huge scale.
Kelly examines the historic records from the times associated with the spread of the pest along the century and indicates a pattern: violent manifestations of the environment appear to accompany the epochs when the pest attacked regions in Europe or in Asia. There is not enough evidence though to conclude that the bacillus causing the pest is fond of environmental instability, but the historic records are often describing natural calamities close to the key moment of the spread of the bubonic disease in both Asia and Europe.
As previously mentioned, another factor that contributed to the successful contamination of two continents, placing them under the threat of being wiped out was warfare. Kelly point out the not even when the plague was at its worst moments in Europe, people did not cease to wage war against one another and give the example of Cola di Rienzo, he calls "possibly the silliest man in Europe" (The Great Mortality, xvi). War lives behind many conditions that encourage the spread of a virus or a bacillus and vehicles like rats and insects, polluted waters and bad air are all thriving in the aftermath of a battle. Kelly analyses the conditions people lived in at the in the early 1300s and points out that the Europe was already confronted by overpopulation, famine was usual, low immune systems were just ready to embrace the attack of the Yesinia Pestis bacillus. Kelly's remarkable analysis goes back and forth in human history comparing results and records, the spreading of different viruses across time and place and undertaking the task to find common features to support his arguments related to the factors that contributed to the high toll the Plague requested from two continents. The advancemtn of science has allowed the discovery of new cures, but also the invention of new increasingly methods to kill. Kelly uses the example of the Japanese who developed a "plague weapon against the Chinese city of Changteh" reported by the U.S. army: "one of Ishiii's greatest achievements…was his use of the human flea, P. irritans. This flea is resistant to air drag, naturally targets humans, and could also infect the local rat population to prolong the epidemic" (The Great Mortality).
Socio-economic conditions, human nature and environmental conditions, ignorance and religion have all contributed to the spread of the bacillus that brought death to large amount of humans and animals in Eurasia. Although the circumstances and exact scientific explanation for the epidemic are still up for debate and research, there is enough evidence to support the idea that humanity is still exposed in front of infectious diseases and the new viruses will always place science under the tremendous weight of being responsible for saving the world from extinction, largely due to its own ignorance.