Bacterial Colonisation of the Nasal and Nasopharyngeal Cavities in Children: The Generation R Study Lebon, A. 2011-03-16 Doctoral Thesis

Humans are surrounded by microorganisms: viruses, bacteria, fungi and parasites. We can divide these organisms into the following four categories: innocent, beneficial, harmful and dangerous. Most of the times, microorganisms are not harmful and are therefore referred to as non-pathogenic. Innocent microorganisms cause no harm nor do they provide benefits. Beneficial microorganisms even provide a significant advantage for humans by aiding digestion or preventing pathogenic microorganisms to cause infection via colonisation resistance. By colonising the respiratory and/or gastro-intestinal tract, these organisms prevent pathogenic microorganisms to settle and cause harm. Pathogens causing harm in certain cases, but not per definition, are grouped into the harmful category. However, microbes that fall into the dangerous category comprise organisms that cause morbidity and mortality in humans even in those with an intact immune system.
http://repub.eur.nl/res/pub/22727/110316_Lebon%2C%20Ankie_Ed.pdf