Our bodies are like mobile warm-blooded coral reefs, rich in microbial biodiversity and home to vast numbers of bacterial cells. Indeed, it has been estimated that there are more bacterial cells (10*14) associated with the average human body than there are human ones (only 10*13)! This normal bacterial flora lives on the external body surfaces & in the gut.

The composition of the normal flora varies somewhat from individual to individual. Some bacterial species may be carried only transiently. Most, however, are fairly permanent--animal experiments have shown that it can be extremely difficult to alter the composition of the normal flora of, say, the gut in a healthy individual. Some members of the normal flora can become pathogenic if they acquire additional virulence factors (e.g. E. coli) or are introduced into normally sterile sites (e. g. Staphylococcus aureus).

The normal flora may even be beneficial:

the normal flora may prevent pathogenic microrganisms from getting a foothold on body surfaces (a phenomenon known as colonisation resistance). Administration of broad-spectrum antibiotics has a profound effect on the normal flora and can result in colonisation with antibiotic-resistant organisms. Antibiotic-mediated disruption of the normal flora can lead to fungal infections, such as superficial or, in the critically ill, invasive candidiasis, or to antibiotic-associated colitis caused by Clostridium difficile.
the normal flora may also produce essential nutrients (e. g. vitamin K is produced by the gut flora)