Microorganisms exist everywhere in the air that we breathe, in the soil, in non-processed foods, dust in all decomposing matter.
Man and animals have micro organisms living both on and in their bodies. Many micro organisms are harmless under ordinary conditions, and some are actually useful. Most plants and animals could not exist without the assistance of micro organisms.
Microorganisms play an important role in the decay process. Microorganisms break down leaves and wood, animals carcasses, etc. City life depends on bacteria for sewage treatment. Micro organisms are used to ferment alcoholic beverages, to make organic acids, to make foods such as yogurt, bread and cheese. Some micro organisms are utilised to produce medical antibiotics. Some microorganisms are harmful, or pathogenic and are responsible for sickness and disease. Microorganisms can damage product such as plastic and adhesives.