How likely is a pandemic?
Epidemiologists believe we're statistically overdue for a global viral outbreak, which occurs every generation or so. This year's Ebola crisis is probably just a dress rehearsal: Though the virus has killed at least 1,420 people in Africa in the last five months, Ebola is transmitted only through intimate contact with bodily fluids and doesn't have the global reach of a true pandemic, such as Spanish influenza in 1918. Humanity had no prior exposure or immunity to the Spanish flu, which is believed to have incubated in birds and pigs. So it spread like wildfire, infecting about 500 million people and killing about 50 million of them. The next pandemic is most likely to emerge in a remote region of Asia or Africa, from contact between people and poultry, rats, bats, pigs, monkeys, or some other animal. If that virus can be spread through the air or by touch, the way the common cold is, it will sweep from village to city, and air travel will allow it to hop continents within hours. A vaccine will take at least months of frantic work to develop, and in the meantime, millions will die. "The three deadliest events in human history were all infectious diseases," says medical historian David Morens: the Spanish flu, the Black Death (bubonic plague), and AIDS. "There are lots of reasons to think more will be coming."

Why are animals the likely source?
About 60 percent of the roughly 400 emerging infectious diseases identified since 1940 are "zoonotic" — originating in animals. Ebola is believed to have originated in bats, and HIV in monkeys, and spread to humans when bushmeat hunters slaughtered the animals, exposing themselves to their blood. "There are 50,000 vertebrate species," says Peter Daszak of Columbia University. "If you assume each one has 20 unknown viruses, that makes a million unknown viruses in wildlife." While these viruses circulate relatively harmlessly within the nonhuman animal kingdom, a "spillover" onto a human with no immunity to the disease can be devastating. Unfortunately, such spillovers are becoming more frequent.

Why is that?
As Earth's population has grown to more than 7 billion, humans have encroached further and further into the habitats of wild animals — unwittingly exposing themselves to a number of unique virulent diseases. The urbanization of Africa and Asia, along with modern air travel, mean that a single infected person can spread a disease to the entire globe. "Up to now, we have been bullet dodging," says virologist Nathan Wolfe. "But right now lurking in the jungle are viruses that, if they crossed over, have the capability to kill hundreds of millions of us."

How are scientists preparing?
Some researchers are focusing on re-creating viruses in the lab, such as the H1N1 virus, known as swine flu. By studying its genetic sequence, epidemiologists hope to be able to understand how such viruses evolve and are transmitted. But experimentation carries risks. One virologist, Dr. Ron Fouchier, has been working with the H5N1 avian flu virus: a strain widely considered the worst-case scenario for a future pandemic because of its high death rate, killing 60 percent of those infected. H5N1 cropped up in 2003 but has yet to spread from one human to another. But in his "Biosafety Level 3" facility in the Netherlands, Fouchier says he has "mutated the hell" out of the virus so that it is now airborne between ferrets — and potentially between humans, too. The U.S. government tried and failed to block Fouchier from publishing his work, out of fear it would be used by terrorists seeking to create a catastrophic biological weapon.

What else is being done?
The research organization Global Viral is trying to create an early-warning system that would detect hidden pandemic viruses at their source and contain them before they get out of control. One of the organization's novel approaches is to employ young social media experts to monitor internet searches and Twitter feeds for such key phrases as "body ache" and "high fever," hoping to catch a virus as it "goes viral" in the newest sense of the term. Virologists, meanwhile, are working on "seed vaccines" for newly emerging strains of influenza.

How can governments help?
First of all, by being transparent when there's an outbreak of a new disease. When Severe Acute Respiratory Syndrome, or SARS, emerged in China in 2002, Beijing officials initially tried to hide the outbreak from visiting World Health Organization experts by ordering doctors to take SARS patients out of hospitals and drive them around in ambulances. The virus eventually killed 775 people across 37 countries. There have been similar delays in reporting cases of the deadly Middle East Respiratory Syndrome, or MERS, in Saudi Arabia in recent years. But the biggest challenge, says infectious-disease expert Mark Woolhouse, is persuading the world's governments to fund research into viruses and prepare for a public health crisis that hasn't yet emerged — but almost certainly will. "We like to think we discover viruses," he says, "but it's also the viruses discovering us."

A pandemic of 'Corrupted Blood'
When a make-believe pandemic hit the largest online role-playing game in the world in 2005, it caught the attention of two epidemiologists at Tufts University. The disease was called "Corrupted Blood," and it infected millions of players across the fantasy universe of World of Warcraft. It was highly contagious, and players who teleported from its source — a dungeon known as Zul'Gurub — carried the infection with them into overcrowded cities. Eric Lofgren and Nina Fefferman have since pored over the Warcraft data to try to understand how people react when caught up in plagues. The answer: recklessly and irrationally. Quarantines were routinely ignored, and players often became exposed to the disease out of carelessness or even curiosity. The scenario wasn't "perfectly realistic," said Lofgren, but it emphasized "the chaos of human reactions to things."