The study of microscopic organisms is called microbiology. Micro-organisms are very small organisms which are not visible through naked eye. These organisms include algae, bacteria, moulds, protozoa, yeasts and viruses. Sometimes they are even referred to as microbes and are studied under microscope, being very small in appearance.
Micro-organisms are almost found everywhere and they constitute by far the largest group of living things existing on earth. Our existence, health, food, agriculture and even industrial progress are very much dependent on them. If microbes were to become extinct, disaster would become inevitable and would be beyond imagination. Unknowingly, man has made use of micro-organism since prehistoric times, mostly in the production of food and drinks, such as leavening of bread, souring of milk and making of wine and beer etc. However, it was only in the seventeenth century that the microbial world was discovered.