Science journalist and blogger Ed Yong explores the physical and philosophical implications of being host to billions of microbes.
He reports the latest science showing how the bacteria we come into contact with can profoundly affect our lives - from the ability to digest different foodstuffs to our susceptibility to asthma, diabetes and even stress and anxiety.
And, he asks whether he should be seen as a human, or a universe of bacteria in a "human shaped sack"?
Four Thought is a series of talks which combine thought provoking ideas and engaging storytelling.
Recorded in front of an audience at the RSA in London, speakers take to the stage to air their latest thinking on the trends, ideas, interests and passions that affect our culture and society.
Producer: Giles Edwards.