Spread of Bubonic Plague ("Black Death")
Spread due to Mongols, merchants, and travelers on trade routes. Thriving in oases and trading cities of central Asia (abudance of domesticated animals).
1346) Reached Black Sea ports of Caffa and Tana.
1347) Italian merchants fled plague infected Black Sea ports, unsurprisingly spreading the disease throughout the Mediterranean basin.
1348) Caused epidemics in most of Europe.
Inflamed lymph nodes, buboes(black swellings).
Killed 60 - 70% of its human victims, strong enough to destroy a society (wiping an entire population).
Long cold winters of Scandinavia prevented fleas and rodents from spreading proliferating. As well as India, in fact, it grew in population.