The disease involved a number of strains that commonly killed 25 to 50 percent of local populations. While climate changes may have contributed to the disease’s spread, it was the Mongol invasions that dispersed it to China and other points around Afro-Eurasia before it began following trade routes to Italy. Reaching European soil, the disease infected rats before killing people. In China an estimated 40 million people perished. Food supplies dropped since the dead and sick could not farm. Cities, with dense populations, lost as much as two-thirds of their population.
