This question originally appeared on Quora. 
Answer by Robyn Correll Carlyle, MPH
Fairly likely.
Larry Brilliant discussed this issue during a 2006 TED talk. In the talk, he said that he had done a study with top epidemiologists. In that study, 90% of them said they thought there would be a pandemic within their children's or grandchildren's lifetimes, where:
- 1 billion people would get sick
- 165 million would die
- There would be a global recession and depression
- and there would be $1-3 trillion cost to the economy
And it's easy to see why. There are a few facets of modern society that make a devastating pandemic not only possible, but likely.
1. We've dramatically increased our global population size. Simply put, there are just more of us now. More of us to get infected. More of us to spread the infection.
Source: Human Conditions
2. We're moving to central locations. So not only are there more of us, but we're moving to places where we are in closer proximity, increasing our risk of spreading diseases to more people, at a faster rate.
This is particularly true in slums, where sanitation services are lacking and many people occupy a small geographical space. While the percent of urban populations living in slums has gone down, it's still very high in parts of the world.
3. (The most important factor): We're traveling farther, faster, and more often. It used to take days, weeks, months to circumnavigate the globe. Now it takes about a day. Very few diseases show symptoms in that short period of a time, so someone could be infected with a virus or bacteria, jump on a plane and infect someone else halfway across the world in a matter of days.
Number of days required to travel the entire distance around the world
The internationalization of Starbucks and McDonalds
Globalization allows us to share goods and diseases.
Trends in international tourism
Only until recently has international travel been feasible for large numbers of people. Traveling has made it easier for diseases to spread farther and more quickly, and it's become more difficult to contain them.
To illustrate: Let's say someone is infected with a novel strain of the flu, one that is spread from human-to-human-to-human. It's not a strain included in the flu vaccine, and antivirals aren't very effective at treating it. The first case pops up in -- say -- a farmer in northern India. That farmer goes to sell goods in New Delhi and comes into contact with another young man. That young man (now infected but not yet showing symptoms) flies out two days later to visit family in England and sits in the middle seat on a tightly-packed airplane. He's very chatty and talks the entire duration of the flight, sharing his respiratory droplets (and consequently the influenza virus) with his seat mates as well as the flight attendant. Upon arriving, the young man goes into the crowded streets of London to visit his family, while his new friends and the flight attendant continue on to their next flights to New York, Rio de Janeiro, and Paris, bringing the virus with them ... Within three weeks, the virus is spreading exponentially on 5 different continents, and health officials are only beginning to notice.
Image from Ready to Take Your Business Global?
You can see how quickly a disease can spread with a little help from modern aviation.
This all assumes a couple things, of course. First, we wouldn't have an effective method of treatment or prevention (i.e. a vaccine) right away. In some cases, standards that medical interventions are required to meet in most countries prior to distribution can be expedited in case of emergencies. But it still takes years (or months, if we're really, really lucky) to develop, test, and approve a new vaccine or treatment. In that time, the disease can continue to spread.
Second, it assumes we aren't able to detect the epidemic in its early stages. Going back to our demonstrative anecdote, say we were able to detect the disease while it was still just in the area around New Delhi. Local health officials could alert clinics and hospitals to watch out for the disease and isolate those who were sick, and track down people who were potentially exposed and quarantine them to ensure that they aren't sick, but more importantly, won't continue to spread the disease.
If an epidemic is caught in its infancy, public health officials can prevent the disease from spreading too far, too quickly, but it takes a good early detection system with international support to make that happen.
We're definitely making improvements in this area. There are some programs out there working toward early detection for early response, such as Innovative Support To Emergencies Diseases and Disasters (INSTEDD) and HealthMap. But these programs are still young in their development.