One of the most dangerous threats to the human population is a simple virus — that is, a deadly disease that spreads rapidly throughout the world. Within the last century we’ve had four major flu epidemics, as well as HIV and SARS, and scientists says it’s inevitable that another will occur. The 1918 influenza outbreak killed more people than World War I, and if a deadly contagion surfaced today, it could spread even faster and infect even more people. Considering how quickly diseases spread though all forms of modern transportation — and the amount of international travel that takes place today — an outbreak similar to that of 1918 “could have a more devastating impact,” says Maria Zambon, head of the Health Protection Agency's Influenza Laboratory.
And if nature doesn’t send such a deadly contagion our way, mankind just might. Biological warfare is another threat that looms over the modern world, and diseases like anthrax, Ebola and cholera have all been weaponized.