The Black Death spread primarily through fleas carried by rats that traveled on trade ships. Originating in Central Asia around 1346, the plague was brought to Europe via Genoese trading ships docking at ports such as Kaffa in Crimea. Fleas infected with the bacterium Yersinia pestis bit rats and humans, transmitting the disease. The rats, common on ships and in cities, facilitated the spread across Europe, the Middle East, and North Africa through trade routes and maritime travel

. Once ashore, the disease spread rapidly both through flea bites (bubonic plague) and person-to-person transmission via respiratory droplets in the case of pneumonic plague, which helped the epidemic move quickly inland

. The plague reached Europe through Mediterranean ports and spread to major cities and rural areas alike, killing an estimated 30% to 60% of the population in affected regions

. In summary, the Black Death spread through:

• Fleas on rats aboard trade ships traveling from Asia to Europe and other regions
• Trade routes connecting Europe, Asia, and North Africa
• Person-to-person airborne transmission of pneumonic plague once established in human populations

This combination of flea-borne and airborne transmission, along with extensive trade and movement of people, caused the rapid and devastating spread of the Black Death across continents in the mid-14th century.