How did the great plague disease spread?

The Great Plague, also known as the Black Death, primarily spread through the bites of infected fleas carried by black rats (Rattus rattus). These rats were common on trade ships and in port cities, facilitating the rapid spread of the disease along trade routes. Upon boarding ships and establishing infestations, the infected fleas could transmit Yersinia pestis - the causative agent of the plague - to rats and humans alike.

When an infected flea bit a human, the bacteria entered the skin and travelled through the lymphatic system to reach nearby lymph nodes, typically causing buboes or painful swellings in the groin, armpits, or neck. In severe cases, the infection entered the bloodstream through a compromised lymphatic system, developing into the septicemic form of the plague; if the lungs get infected it could lead to pneumonic plague and be transmitted through air between humans.

This process contributed to the disease's widespread transmission and devastating impact during its first and subsequent pandemics throughout the medieval world from the mid-14th century onwards. Trade and travel, especially through ports of major trade cities, further aided the spread of the disease across great distances and facilitated contact with the vectors and sources of infection.