The flea is parasitic on house and field rats and seeks out other prey when its rodent hosts die. Rats were an amplifying factor to bubonic plague due to their common association with humans as well as the nature of their blood. The rats blood allowed the rat to withstand a major concentration of the plague. The bacteria form aggregates in the gut of infected fleas and this results in the flea regurgitating ingested blood, which is now infected, into the bite site of a rodent or human host. Once established, bacteria rapidly spread to the lymph nodes and multiply. The fleas that transmit the disease only directly infect humans when the rat population in the area is wiped out from a mass infection. Furthermore, in areas of a large population of rats, the animals can harbor low levels of the plague infection without causing human outbreaks. With no new rat inputs being added to the population from other areas, the infection would only spread to humans in very rare cases of overcrowding. Signs and symptoms
After being transmitted via the bite of an infected flea, the Y. pestis bacteria become localized in an inflamed lymph node, where they begin to colonize and reproduce. Infected lymph nodes develop hemorrhages, which result in the death of tissue.Y. pestis bacilli can resist phagocytosis and even reproduce inside phagocytes and kill them. As the disease progresses, the lymph nodes can hemorrhage and become swollen and necrotic. Bubonic plague can progress to lethal septicemic plague in some cases.
Chinese records also showed a huge outbreak in Mongolia in the early 1330s.In 2022, researchers presented evidence that the plague originated near Lake Issyk-Kul in Kyrgyzstan. The Mongols had cut the trade route (the Silk Road) between China and Europe, which halted the spread of the Black Death from eastern Russia to Western Europe. The European epidemic began with an attack that Mongols launched on the Italian merchants last trading station in the region, Caffa in the Crimea.In late 1346, plague broke out among the besiegers and from them penetrated the town. The Mongol forces catapulted plague-infested corpses into Caffa as a form of attack, one of the first known instances of biological warfare. When spring arrived, the Italian merchants fled on their ships, unknowingly carrying the Black Death. Carried by the fleas on rats, the plague initially spread to humans near the Black Sea and then outwards to the rest of Europe as a result of people fleeing from one area to another. Rats migrated with humans, traveling among grain bags, clothing, ships, wagons, and grain husks. Continued research indicates that black rats, those that primarily transmitted the disease, prefer grain as a primary meal. Due to this, the major bulk grain fleets that transported major citys food shipments from Africa and Alexandria to heavily populated areas, and were then unloaded by hand, played a role in increasing the transmission effectiveness of the plague. Third pandemic
The plague resurfaced for a third time in the mid-19th century; this is also known as "the modern pandemic".