The answer in terms of terrestrial means of such an event would be severe earth quake and volcanic eruption, such as the huge one at Yellow Stone National Park which is said to be about 55KM by 72Km and is one of the biggest super volcanoes in the world. If such an eruption did occur, thousands of people would be killed by the hot ash that would rain down over 1,000 KM and cause Nuclear Winter which would have a devastating effect on all life on this planet, as we know it.Scientists would have warning signs well ahead of time. as they are monitoring the volcano very carefully, and say it is not likely to occur soon. Extraterrestrial events, such as a huge asteroid, would certainly cause world wide devastation and could occur without much warning. We are all living on a very fragile planet under risk of great devastation at any time, unless the powers of our God intervene and keep us all safe for another 50,000 years or so. Viruses are certainly a worry, yet so far we have contained and even eradicated some of the worst. New ones that are immune to anti-viral injections, are of concern and these may eventually take hold in the future if we are not more conscious about pollution and monitoring causes that might lead to such a calamity. If people on his planet were concerned as they should be, perhaps some of the sources of a calamity might be prevented with more care and policing of our natural resources and their uses. The Disease control center in Atlanta is continually working on, and trying to create new anti-virus solutions.As always, it is up to us as human kind, to safe guard and plan to deal with anything that poses a threat to massive casualties on this planet.