In 1674, Anton van Leeuwenhoek discovered a world of millions of tine “animalcules”. About 100 years later, Otto Muller, the Danish biologist, extended Leeuwenhoek’s studies and organized bacteria into genera and species according to a classification method Carolus Linnaeus. This was the beginning of the taxonomic classification of microbes. Robert Koch and Louis Pasteur- 1870s and 1880s with a series of experiments that microorganisms were responsible for causing anthrax, rabies, plague, cholera, and tuberculosis. Others proved they were also responsible for causing human disease. This is pretty much the germ theory of disease. Etiology - is the study of the causation of disease. gram-staining - developed in the 1880s and is still used today to differentiate bacteria into two categoriesgram-positive gram-negative cowpox-based vaccine for preventing smallpox began interest in the studies in the field of immunology by Jenner. Ehrlich laid the foundation for the field of chemotherapy by his search of “magic bullets”- Microbes can be subdivided into the following four groups Viruses Fungi Bacteria Parasites •Smallest infectious particles from 18-300 nanometers •Consist of DNA or RNA and may contain protein for replication and pathogenesis; components are then enclosed in a protein coat with or without a lipid membrane coat. •Parasites- requiring host cell to replicate •The cells they infect and the host response to the infectious dictate the nature of the clinical manifestation. •Infection can lead to rapid reproduction and destruction of the cell or to a long-term chronic relationship with possible integration of the viral genetic information into the host genome. •Prokaryotic organisms •Simple unicellular organisms with •no nuclear membrane, mitochondria, Golgi bodies, or endoplasmic reticulum•reproduce by asexual means. •Consist of two basic formsgram-positive cell wall - thick peptidoglycan layer gram-negative- thin peptidoglycan layer and an overlying outer membrane. •The human body is inhabited by thousands of different bacterial species. •Many things surrounding us are also inhabited by bacteria. •Disease result from the toxic effects of bacterial products (toxins) or when bacteria inhabit normally sterile body sites •Eukaryotic •Well-defined nucleus • mitochondria •Golgi bodies •endoplasmic reticulum can exist in either unicellular (fungi) that can replicate asexually or in a filamentous form (mold) that can replicate asexually or sexually. •Some can assume either morphology and are known as dimorphic fungi. •Most complex •All are considered eukaryotic • unicellular •multicellular •Life cycles are very complex •Difficulty is not just understanding the spectrum of disease caused by parasites, but also an appreciation of the epidemiology of these infections. Microbiology now a days is more used on answering questions regarding biochemistry which is the study of metabolism. This extends toward the cell function at the molecular level. From applied knowledge through basic research, scientist can answer questions also regarding recombinant DNA technology and gene therapy.