This special edition of the eLife Podcast marks our 50th episode and we've decided to mark the milestone by focusing on a field that's huge and tiny both at the same time: huge in terms of the rate at which the discipline's growing and the impact it's set to have our lives, and tiny because its subjects are microscopic. It's our microbiome, the community of micro-organisms that live on us and in us and outnumber our own human cells by maybe 50 fold: we're literally passengers in our own bodies, and over the next 30 minutes we'll hear how gut bacteria might alter your risk of diabetes, and how the microbiome can manipulate your mood...