What makes an idea, a brand, or a behavior catch fire? On this week's show, TED speakers explore the mysteries behind the many things we spread: laughter and sadness, imagination, viruses and viral ideas.

Neuroscientist Sophie Scott studies laughter, specifically its effect on our body and brain. She discusses laughter's contagious nature, as well as its role in maintaining social bonds.

Entrepreneur and blogger Seth Godin describes how the marketing of ideas has changed since the invention of sliced bread, as well as the type of ideas that stick in consumer's minds

Entrepreneur and philanthropist Bill Gates stresses our dire need for a system that can take on the challenges of the next global health epidemic.

Physician and social scientist Nicholas Christakis explains how face-to-face social networks and their structures influence behaviors and phenomena in human society and the natural living world.

Historian Yuval Noah Harari explains how human imagination powered the growth and spread of homo sapiens around the world. (11:00)