What makes an idea, a brand, or a behavior catch fire? This hour, TED speakers explore the mysteries behind the many things we spread: laughter and sadness, imagination, viruses and viral ideas. TED speakers include Microsoft founder Bill Gates, neuroscientist Sophie Scott, entrepreneur and blogger Seth Godin, physician and social scientist Nicholas Christakis, and historian Yuval Harari. (Original Broadcast Date: March 04, 2016)