Please Explain is all about the Norovirus, which is the leading cause of outbreaks of food-borne illness. We'll speak with Dr. Aaron Margolin and Dr. Christine Moe.

This week’s Please Explain takes a look at something familiar (yet still mysterious) to every New Yorker: the subway. John Tauranac, architecural historian and designer of city and transit maps, and Andrew Sparberg, former Long Island Railroad manager and director of the railroad technology program at Technical Career Insitutes, talk about how the subway was built and how it transformed the metropolitan area.

This week's Please Explain is the final installment of our series How to Save the World. Jeffrey Sachs discusses whether it's possible to achieve world peace. He's Director of The Earth Institute at Columbia University and Special Advisor to United Nations Secretary-General Ban Ki-moon. His most recent book is The Price of Civilization.

This week's Please Explain continues our series How to Save the World. Glenn Denning, Director, Center on Globalization and Sustainable Development at the Earth Institute at Columbia University, and J. Matthew Roney, Research Associate, Earth Policy Institute, examine the global food supply.

Wherever humans go, they leave trash behind. The average American throws away over 1,130 pounds of waste per year. On this week’s Please Explain, we continue our series How to Save the World, looking at how we dispose of garbage, how recycling and composting and smaller packaging can cut down on the amount of garbage people throw away around the world, and how garbage can be used as a renewable, green energy source. Joining us are Nickolas J. Themelis, Director, Earth Engineering Center, and professor in the School of Engineering at Columbia University, and Elizabeth Royte, author of Garbage Land and Bottlemania.

This week’s Please Explain, the third in our series How to Save the World, is about climate change and how to stop it. David Archer, professor of geophysical sciences at the University of Chicago, and author of The Long Thaw: How Humans are Changing the Next 100,000 Years of the Earth’s Climate, and Global Warming: Understanding the Forecast; and Klaus Lackner, Director of the Lenfest Center for Sustainable Energy at Columbia University’s Earth Institute join us to talk about carbon in the atmosphere, how and why it is causing climate change, and how to slow or stop climate change by using sustainable energy and carbon sequestration.

This week's Please Explains is the second in our series on how to save the world—ways to approach complex global problems such as climate change, food supply, garbage disposal, the global water supply, and violence. Today we're looking at the population explosion—there are now 7 billion people on the planet. We're joined by Hania Zlotnik, director of the population Division at the Department of Economics and Social Affairs at the United Nations, and Dr.Joel E. Cohen, mathematical biologist and the head of the Laboratory of Populations at Rockefeller University and Columbia University, and author of How Many People Can the Earth Support?

We're kicking off a series of Please Explains on how to save the world—ways to approach complex global problems such as climate change, food supply, garbage disposal, population control, and violence. Today's topic is how to protect the world's water supply. Upmanu Lall, Director of the Columbia Water Center, and Sandra Postel, founder of the Global Water Policy Project and National Geographic Freshwater Fellow join us to discuss the state of fresh water around the globe.

Tuberculosis remains one of the world’s deadliest diseases—accounting for 9.4 million cases and 1.7 million deaths in 2009, according to the WHO. Maryn McKenna, science journalist and author of Superbug, and Dr. Neil Schluger, Professor of Medicine, Epidemiology, and Environmental Health Sciences at Columbia University Medical Center and Chief Scientific Officer for the World Lung Foundation, give us a history of the disease, how it spreads, why it’s so hard to treat, and how drug-resistant TB has emerged and what it means for the future of treating the deadly disease.

Almost everyone has closets full of stuff, favorite mementos, and expanding collections of books or shoes or spices or hotel shampoos. But sometimes our emotional attachments to stuff can spiral out of control, and people become not just pack rats but compulsive hoarders. Dr. Robin Zasio, therapist who specializes in treating hoarding and other anxiety-related disorders, explains what compulsive hoarding is and how to treat it. She’ll also give advice about how to live a less-cluttered, better-organized life. She’s the author of The Hoarder in You: How to Live a Happier, Healthier, Uncluttered Life.