"Black Death" refers to a 14th-century outbreak of the bubonic plague, a bacterial infection spread primarily by infected fleas, though the disease can also be transmitted by person-to-person contact in its pneumonic form. According to About.com, the epidemic began with the fall of Kaffa, a Venetian colony in the Crimea.
Refugees from the siege of Kaffa evacuated their city and fled back to Europe on ships that carried black rats. These rats played host to fleas with the bacterium Yersinia pestis in their guts. On being bitten, animals and humans alike became infected and began to develop the classic symptoms of bubonic plague. As About.com reports, these symptoms included large black swellings at the site of inflamed lymph nodes and dark, mottled spots on the skin.
If the infection spread to the victim's lungs, the disease could develop into its more virulent form and become airborne. This pneumonic plague had the capacity to spread from person to person through the air, and it was often responsible for the telltale bad breath of the dying victim. According to About.com, the Black Death is regarded as the worst natural disaster ever to have stricken Europe, and it likely killed up to half of the population.