As the circumstances for the survival and growth of microorganisms are comparable to those of the human beings, they exist in air we breathe and the food we consume. Microorganisms may be found everywhere in nature, including in humans, seas, mountains, and lakes. Our bodies are covered with them, as well as in our mouths, nostrils, and digestive tracts. To our relief, the vast majority of microorganisms pose no threat to human beings, and we have developed defences against the few that may be dangerous. Because of their unique properties, microorganisms serve as excellent models for study of a wide range of underlying processes. Whether in a bacteria, a mouse, or a human, many life processes take place at the cellular level in the same way. Although many fungal, lichen, and algal growth forms deviate from the definition of microbiology, which is often associated with organisms that cannot be seen without a microscope. Though it is still fungus by definition, the Armillaria associated with the tree roots in a number of regions has a community structure that is measured in hectares.