Risks come in many forms and are occurring now much more frequently and on a larger scale. A pandemic is one of the most dangerous; a highly infectious disease that spreads across a vast area, usually multiple continents or even worldwide. The deadliest recorded was the Black Death in the 14th Century, which killed an estimated 75 to 200 million people.
Among the most prevalent forms of pandemic throughout history have been HIV/AIDS, cholera, influenza, typhus, smallpox and measles. Others include tuberculosis, leprosy, malaria and yellow fever. Although the threat has greatly receded thanks to advances in medicine, they often come back in new and far deadlier strains.