ABSTRACT

Microbes are organisms that are too small to be seen without using a microscope; such as bacteria, viruses, archaea, fungi and protists. There is enough evidence in the literature to believe that microorganisms were the earliest of the living things that existed on this planet (Waksman, 1959). Since time immemorial, man has taken advantage of the activities of the microbes without understanding their true nature and without appreciating their role in living processes. This is true for such important processes as the raising or the leavening of bread, the souring of milk, the making of wine and spoilage of wine etc. (Kluyver and van Nile, 1956). it was not, however, until the 19th century that the true nature of the microbes, their role in the causation of humans, animals and plant diseases, on one hand, and in various fermentations and in soil processes, on the other hand, were gradually recognized (Alcamo, 1983). The credit for discovering and describing microorganisms goes to the Dutch Draper Anton Van Leeuwenhoek (1632 – 1723), who is known as the Father of Microbiology and whose hobby was to grind tiny lenses, build microscopes and examine ‘animalcules’ from waste material, human saliva, sewage water etc. Thus he introduce man, through his microscope, to a new world of animalcules (little animals) because of their frequent motility The contributions of Louis Pasteur, J.H. Villemin, Robert Koch and E. Metchnikoff to our understanding of the role of microbes in the causation of human and animal diseases laid the basis for our knowledge of microbes as the agent responsible for infections and epidemics that have plagued mankind since time immemorial, as well as methods of prevention and therapy. Microbiologists are concerned with microscopic living forms which comprise plants, animals and primitives. Based on the applied aspect of Microbiology, it can be divided into Agricultural Microbiology; Industrial Microbiology; Food Microbiology and Medical/Veterinary Microbiology which deals with microorganisms causing diseases of man and animals. Germ theory postulated by Louis Pasteur is an evidence that human, animal and plant diseases are caused by microorganisms known as pathogens or germs and some of them are contagious, scientific explanation came for the first time from Louis Pasteur. These small organisms, which are too small to be seen without magnification, invade animals, plants and even bacteria. Their growth and reproduction within their hosts can cause disease. Infectious Agents: The agents of infection can be divided into different groups on the basis of their size, biochemical characteristics, or manner in which they interact with the host. Diseases caused by pathogens are called infectious diseases. Even when a pathogen is the principal cause of a disease, environmental and hereditary factors often influence the severity of the disease, and weather a potential host individual becomes infected when exposed to the pathogen. Pathogens are diseasecausing agents that can pass from one individual to another, across multiple domains of life – including bacteria, viruses, fungi and parasites – as well as the reaction of tissues to their presence or to the toxins that they produce.