Microbiology is the study of microorganisms, which are unicellularor cell-cluster microscopicorganisms. This includes eukaryotessuch as fungiand protists, and prokaryotessuch as bacteriaand certain algae. Viruses, though not strictly classed as living organisms, are also studied. Microbiology is a broad term which includes many branches like virology, mycology, parasitologyand others. A person who specializes in the area of microbiology is called a microbiologist.

Although much is now known in the field of microbiology, advances are being made regularly. We have probably only studied about 1% of all of the microbes on Earth. Thus, despite the fact that over three hundred years have passed since the discovery of microbes, the field of microbiology could be said to be in its infancy relative to other biological disciplines such as zoology, botanyand entomology.

The field of microbiology can be generally divided into several sub disciplines:

• Microbial physiology: The study of how the microbial cell functions biochemically. Includes the study of microbial growth, microbial metabolismand microbial cell structure.
• Microbial genetics: The study of how genesare organized and regulated in microbes in relation to their cellular functions. Closely related to the field of molecular biology.
• Medical microbiology: The study of the role of microbes in human illness. Includes the study of microbial pathogenesisand epidemiologyand is related to the study of disease pathologyand immunology.
• Veterinary microbiology: The study of the role in microbes in veterinary medicineor animal taxonomy.
• Environmental microbiology: The study of the function and diversity of microbes in their natural environments. Includes the study of microbial ecology, microbially-mediated nutrient cycling, geomicrobiology, microbial diversity and bioremediation. Characterisation of key bacterial habitats such as the rhizosphereand phyllosphere, soiland groundwaterecosystems, open oceansor extreme environments (extremophiles).
• Evolutionary microbiology: The study of the evolution of microbes. Includes the study of bacterial systematicsand taxonomy.
• Industrial microbiology: The exploitation of microbes for use in industrial processes. Examples include industrial fermentationand wastewater treatment. Closely linked to the biotechnologyindustry. This field also includes brewing, an important application of microbiology.
• Aeromicrobiology: The study of airborne microorganisms.
• Food Microbiology: The study of microorganisms causing food spoilage.
• Pharmaceutical microbiology: the study of microorganisms causing pharmaceutical contamination and spoilage.

The existence of microorganismswas hypothesized during the late Middle Agesbut they were not observed or proven until the invention of the microscopein the 17th century. In The Canon of Medicine(1020), Abū Alī ibnSīnā (Avicenna)stated that bodily secretionis contaminated by foul foreign earthly bodies before being infected, but he did not view them as primary causes of disease. When the Black Deathbubonic plaguereached al-Andalusin the 14th century, Ibn Khatima and Ibn al-Khatib hypothesized that infectious diseasesare caused by microorganisms which enter the human body.

Bacteriawere first observed by Anton van Leeuwenhoekin 1676 using a single-lens microscopeof his own design. The name "bacterium" was introduced much later, by Ehrenbergin 1828, derived from the Greekβακτηριον meaning "small stick". While van Leeuwenhoek is often cited as the first microbiologist, the first recorded microbiological observation, that of the fruiting bodies of molds, was made earlier in 1665 by Robert Hooke.

The field of bacteriology(later a subdiscipline of microbiology) is generally considered to have been founded by Ferdinand Cohn(1828–1898), a botanist whose studies on algaeand photosynthetic bacterialed him to describe several bacteria including Bacillusand Beggiatoa. Cohn was also the first to formulate a scheme for the taxonomic classificationof bacteria. Pasteur(1822–1895) and Robert Koch(1843–1910) were contemporaries of Cohn’s and are often considered to be the founders of medical microbiology. Pasteur is most famous for his series of experiments designed to disprove the then widely held theory of spontaneous generation, thereby solidifying microbiology’s identity as a biological science. Pasteur also designed methods for food preservation (pasteurization) and vaccines against several diseases such as anthrax, fowl cholera and rabies. Koch is best known for his contributions to the germ theory of disease, proving that specific diseases were caused by specific pathogenic microorganisms. He developed a series of criteria that have become known as the Koch's postulates. Koch was one of the first scientists to focus on the isolation of bacteria in pure cultureresulting in his description of several novel bacteria including Mycobacterium tuberculosis, the causative agent of tuberculosis.

While Pasteur and Koch are often considered the founders of microbiology, their work did not accurately reflect the true diversity of the microbial world because of their exclusive focus on microorganisms having direct medical relevance. It was not until the work of Martinus Beijerinck(1851–1931) and Sergei Winogradsky(1856–1953), the founders of general microbiology(an older term encompassing aspects of microbial physiology, diversity and ecology), that the true breadth of microbiology was revealed. Beijerinck made two major contributions to microbiology: the discovery of virusesand the development of enrichment culture techniques. While his work on the Tobacco Mosaic Virusestablished the basic principles of virology, it was his development of enrichment culturingthat had the most immediate impact on microbiology by allowing for the cultivation of a wide range of microbes with wildly different physiologies. Winogradsky was the first to develop the concept of chemolithotrophyand to thereby reveal the essential role played by microorganisms in geochemical processes. He was responsible for the first isolation and description of both nitrifyingand nitrogen-fixing bacteria.

Virology, often considered a part of microbiologyor of pathology, is the study of biological viruses and virus-like agents: their structure and classification, their ways to infect and exploit cellsfor virus reproduction, the diseases they cause, the techniques to isolate and culture them, and their potential uses in research and therapy.

Mycobacterium tuberculosis is the bacterium that causes most cases of tuberculosis.