What does it mean when people say ‘we’re due for a pandemic’?

There have been warnings over the past 15 years or so that the world is due for a pandemic. Epidemiologically, pandemics are animal viruses that need to be able to do three things: efficiently infect humans, replicate in humans and spread easily among humans. Are there aspects of the globalized society we live in today that have been contributing to those conditions? Marco Werman speaks with science journalist Sonia Shah, author of “Pandemic: Tracking Contagions from Cholera to Ebola and Beyond.”