To understand that we need to first understand where the start epidemic is simply an outbreak of infection in a localized area of pandemic is when that infection spreads to multiple countries around the world, and while on one hand we're better protected than before, on the other are changing.

Society brings together a host of other challenges.

In this day and age, all you need is one person at the airport, and you could infect the whole world.

The majority of pandemics to date have been strains of influenza.

Pandemics can be any type of infection, usually a virus.

Many new or novel viruses come from animals they can pass on to humans after close contact and a process called zoonotic spill over.

And when that happens, sometimes the virus is not very transmittable because it suited to circulating only in animals.

But sometimes we see once that can transmit from person to person, and they can be quite efficient at doing this.

And in that case, an outbreak can become an epidemic as it grows, and there's a chance that it could spread more widely.

This is what happened in the 2009 swine flu pandemic, a pig influenza virus moved into people.

Pig viruses are similar to human viruses on that allowed human to human transmission toe happen quicker.

More than a decade is passed since the 2009 pandemic, and the world's population has grown, as have the risks of passing viruses between people.

We're getting more and more people on the planet's having more and more globalization people traveling from country to country more than ever before.

So these are the kind of things that will cause pendants, toe happen.

Medicine, technology and communication have all improved with time on serious outbreaks require international cooperation.

But experts say the real question isn't if a new pandemic strikes but instead how bad it could be.