The science of microbiology is concerned with a systematic study of microorganisms , notably very small unicellular or multicellular organisms ( typically between 0.01 - 70 micrometresize ) :- e.g. the EUCARYOTICS such as fungi ( including moulds & yeasts) , algae , protozoa , helminths # and the PROCARYOTICS , bacteria , mycoplasma , rickettsiae , chlamydiae # ; plus non-cellular viruses . Anton van Leeuwenhoek in Holland was the first man to discover microorganisms, - which he did in 1674 in consequence of his invention of an optical microscope capable of magnifying by > 200x . French biologist 'Louis Pasteur' is credited with uncovering the role of bacteria in fermentation and disease whilst working to prove a 'germ theory of disease' during the 1860s ; and the German Nobel Laureate Robert Koch is credited with some of the earliest effective research into bacteriology during the 1870s. Koch discovered the role that Bacillus Anthracis rods and spores have to play during the development of the infectious anthrax disease , and this was the first time that the causative agent of an infectious disease had been demonstrated beyond reasonable doubt. Robert Koch went on to isolate the bacilli that were causative agents of tuberculosis , cholera and other diseases.         

                       Major advances in microbiology during the twentieth century have enabled a multitude of disease-causing microorganisms to be identified , and the means of controlling their harmful effects to be developed. Research has uncovered effective means of channelling the activities of various microorganisms so as to benefit medicine , industry and agriculture. Moulds are employed to produce enzymes and antibiotics , notably penicillin. Certain bacteria are cultivated on a large scale for use in the commercial production of lactic acid , the derivatives of which are used as a means of treating anaemia and calcium deficiency. The field of genetic engineering has led to entirely new industrial applications for bacteria , since some of those genetically altered organisms are capable of synthesizing insulin or other biochemicals of medical value.                                              

                      It is said that infectious diseases spread by microorganisms have been a deadlier enemy to mankind than has war. Of 300,000 crusaders who left West Europe for the Holy Land in A.D. 1096 , more than 90% are thought to have died from disease before reaching Jerusalem. Bubonic plague destroyed a third of medieval Europe's population. Napoleon Bonaparte lost 300,000 soldiers through typhus during the Peninsular War . In the Crimean war cholera , smallpox and typhus killed ten times as many British soldiers as were killed in action.                                            

                      However, many benefits are derived by humans from the symbiotic relationship established with their normal microbial population flora. For example , the nutrients biotin, pyridoxine, pantothenic acid , vitamins K and B12 are obtained in part from excesses secreted by coliform bacteria. The Escherichia coli intestinal bacteria live in a mutualistic symbiotic relationship with their human host. They derive nutrients from food materials ingested by man, and produce vitamin K which is an essential factor needed in blood-clotting. Merely by occupying a place and using nutrients present - the normal flora prevent other potentially pathogenic organisms from establishing a site of infection. Indigenous microbes provide a continuing source of irritants and antigens stimulating the immune system, which is therefore fitter to respond effectively by rapidly producing antibodies to other foreign potentially disease-producing agents that may appear. Normal human flora can include bacteria, fungi, protozoans, viruses etc. - different species of which are found on the skin, in the mouth, nose, respiratory tract, eye and ear, intestine and urogenital tract.