In 1976 Peter Piot was a 27-year-old microbiologist working in Belgium when he travelled to the Democratic Republic of Congo, then called Zaire, to investigate a particularly deadly disease outbreak. He took samples back to his lab and was among the team that first discovered the ebola virus.

Today, he is one of the world's leading experts on epidemics and infectious diseases. This includes HIV/AIDS. In 1995, he was the founding director of the United Nations Program on AIDS, called UNAIDS, and served in that role until 2008. He is now the director of one of the world's most prestigious health research institutes, the London School of Hygiene and Tropical Medicine.

And on the podcast today, we talk about epidemics and what can be done to avert and contain them. This includes the ongoing ebola epidemic in the eastern part of the Democratic Republic of the Congo,which is now the second worst ebola outbreak in history. And we also discuss what the world has gotten right -- and wrong about both fighting HIV and AIDS and how we define ending the HIV/AIDS epidemic.

We kick off though discussing the kind of nightmare scenarios that most concern Peter Piot. This includes what he calls "the big one."