Around 1337, a new threat emerged: disease. In Issyk-Kul, a trading post along the Mongol routes, the bubonic plague began. The bacteria, carried by fleas, caused painful buboes and rapid death, as seen in the case of Kutluk and his wife. The disease spread rapidly, facilitated by improved transportation and the prevalence of black rats and their plague-infected fleas on trade routes.

In 1215 AD, Genghis Khan leads 50,000 Mongol warriors to conquer Chengdu, China. His rise from exile to uniting the Mongols and expanding his empire was driven by a fierce desire to prove himself. The key to his military success was the horse, enabling rapid movement and devastating blitzkrieg tactics. Mongols started riding at age three, could shoot at full gallop, and covered vast distances, moving faster than news of their arrival.

A surge in solar activity caused global warming and drought in Mongolia, forcing the Mongols to expand southward. China, with its vast resources, became the ultimate prize. Genghis Khan offered cities an ultimatum: surrender or die, establishing a reputation for extreme cruelty, including massacres and towers of skulls. It's estimated that one in 200 people alive today carry his genes due to his widespread rapes.

The Mongols, despite their cavalry, struggled with fortified cities. They forced Chinese engineers to build battering rams, using prisoners of war to attack their own city walls. Chengdu fell, resulting in over 100,000 deaths and the city being torched. In his lifetime, Genghis Khan was responsible for the deaths of up to 40 million people, conquering a larger empire in 25 years than Rome did in 400. His empire's success was also built on a sophisticated communication system, a prototype of the Pony Express, which facilitated trade and the spread of innovations like paper, printing, and gunpowder from East to West.

The plague reached Caffa, a vital port controlled by Italian merchants. Jani Beg, a descendant of Genghis Khan, besieging the city, weaponized the plague by catapulting infected corpses into Caffa. This early form of biological warfare spread the disease rapidly, forcing the inhabitants to flee, inadvertently carrying the plague with them to Europe.

In 1348, the plague arrived in Sienna, Italy. Agnolo di Tura's account details his family's desperate attempts to ward off the disease, unaware of its true cause. Europe, with its dense, unsanitary cities and large population, provided an ideal environment for the plague to thrive. Rats and their fleas were commonplace. The disease mutated, becoming airborne and even deadlier, with a nearly 100% kill rate.

Sienna lost 60% of its population in six months. Agnolo di Tura himself buried his children. The immense death toll led many to believe it was the end of the world, fostering widespread fear and panic. The predominant belief was that the plague was divine punishment, an avenging God. Pope Clement VI in Avignon, despite his power and wealth, could do nothing to stop the plague, which killed 1,300 people in Avignon in a single day. His consecration of the River Rhone as a floating cemetery further underscored humanity's helplessness, leading to a profound loss of faith in traditional authorities.

Fear and rage led to the search for a scapegoat. In Strasbourg, Germany, rumors spread that Jewish communities were poisoning wells. In the absence of scientific understanding, superstition and prejudice fueled animosity. Despite attempts by authorities to protect them, the mob took matters into their own hands. On February 14th, 1349, known as the St. Valentine's Day Massacre, 1,000 Jews were burned alive after being given the choice to convert or die. However, this act did not save the city, as the plague arrived five months later, claiming another 16,000 victims.

The plague killed over 50 million people across Asia and Europe in 15 years, leaving cities deserted. The Atlantic Ocean, however, protected the Americas from the initial wave of the plague. In this isolated 'New World,' the Inca warrior Pachacuti emerged as a visionary leader. Despite lacking horses, iron tools, and wheeled vehicles, the Incas developed advanced high-altitude agriculture, cultivating vital crops like potatoes, tomatoes, and corn, which would later be introduced to Europe by the Spanish. Pachacuti, driven by a vision of the Sun God Inti, faced the fearsome Chanka warriors, who fought alongside the mummified body of their dead king, Uscovilca. Pachacuti strategically aimed to capture Uscovilca's mummy, believing it would break the Chankas' morale. Through his leadership and the belief in Inti, Pachacuti rallied his men. His bravery in leading the charge secured a legendary victory for the Incas, creating the largest empire in the Americas, encompassing much of modern-day Chile, Bolivia, and Peru, connected by a vast trail network leading to wonders like Machu Picchu. However, the Americas' isolation was nearing its end, as plague survivors in Europe were poised for a new era of exploration and discovery.