Microbial Ecology 1.
Animals are of a similar magnitude.
The vast majority of the bacteria are located in the gastrointestinal tract. Since most individuals are not sick, these organisms must not be causing disease. While there are circumstances when normal flora can cause disease, in a well functioning individual this is not the case. When an individual is compromised and the normal flora gets access to a new habitat, disease may occur. These so called opportunistic infections occur when there is selective removal of normal flora from a given habitat thus allowing access by other normal flora but which are not normally found at that site.