In utero(still in the uterus) mammals are as sterile as any internal organ, which is to say, nearly if not entirely so. Other than that there is this one condition out there where you don't have any normal flora and it is called death. It is not plausible, as of today, for anyone to survive without normal flora. As the typical number of normal flora that we would expect to be present decreases, the chance/rate for infection generally increases. If the levels of normal flora within your body get to low you wont be fighting just one infection but many different infections throughout your body. Infections can form most anywhere in your body that is not sterile, from somewhere on your skin to anywhere throughout your gastrointestinal tracts. Also, there are a few hundred different species of bacteria in the gut that are generally beneficial. They synthesize vitamins such as folic acid, vitamin K and biotin, and ferment complex indigestible carbohydrates.
I hope this helps, cheers!
Matty T!