You have a hundred trillion of these guys in you right now. Before you were born, you had hardly any.
Back then, you were floating in amniotic fluid, protected, sanitized. Bacteria kept their distance. Until you slipped down that birth canal, you were pretty much spic-and-span.
Then came your birthday, and all of a sudden, you were invaded. From the delivery, from the doctor’s hands, from the first meal at your mother’s breast, from your older sister who kisses you (or, if no one’s looking, spits on you), from everyone, from everywhere came an army of bacteria that moved in and stayed.
Link: Gut Bacteria Know Secrets About Your Future : Krulwich Wonders… : NPR.