A microorganism or microbe is a microscopic organism, which may be single-celled or a cluster of cells. 

The possible existence of unseen microbial life was suspected from ancient times, such as in Jain scriptures from 6th century BC India, and the 1st century BC book "On Agriculture" by Marcus Terentius Varro. Microbiology, the scientific study of microorganisms, began with their observation under the microscope in the 1670s by Antonie van Leeuwenhoek. In the 1850s, Louis Pasteur found that microorganisms caused food spoilage, debunking the theory of spontaneous generation. In the 1880s Robert Koch discovered that microorganisms caused the diseases tuberculosis, cholera, and anthrax. 

Microorganisms include all unicellular organisms, and so are extremely diverse. Of the three domains of life identified by Carl Woese, all of the Archaea, and Bacteria are microorganisms. These were previously grouped together in the two domain system as Prokaryotes, the other being the eukaryotes. The third domain Eukaryota includes all multicellular organisms, and many unicellular protists and protozoans. Some protists are related to animals and some to green plants. Many of the multicellular organisms are microscopic, namely micro-animals, some fungi and some algae, but these are not discussed here. 

They live in almost every habitat from the poles to the equator, deserts, geysers, rocks, and the deep sea. Some are adapted to extremes such as very hot or very cold conditions, others to high pressure and a few such as "Deinococcus radiodurans" to high radiation environments. Microorganisms also make up the microbiota found in and on all multicellular organisms.
From where to where?
poles to the equator, deserts, geysers, rocks, and the deep sea