Building on this article, for those who are interested in some science on the subject, what you eat greatly influences your biome. A vegan diet has been shown to substantially change your flora [1] in as little as a day [2]. I also found this video on the role of fiber, our gut flora, and our immune system interesting [3].

[1]: http://www.nature.com/ejcn/journal/v66/n1/full/ejcn2011141a....

[2]: http://www.nature.com/nature/journal/v505/n7484/full/nature1...

[3]: http://nutritionfacts.org/video/prebiotics-tending-our-inner...