All the microscopic organisms that reside on the body of any other animal, human being, or some inanimate object without causing any infection are referred to as normal flora.
Since these normal florae need nutrients and a place to live and survive thus they prevent any other pathogen to colonize the body's surface.
Some of the normal flora is extremely beneficial for us such as those living in the oral cavity, the skin, intestine, and the vaginal epithelium.
Normal flora can also become pathogenic:
In case of a weak immune system, these normal florae could become pathogenic thus, causing diseases.
When these normal florae move outside to their normal habitat, they might cause infection.
For example, an organism living in the intestine might cause a urinary tract infection.