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Microorganisms were the first living creatures on earth. It is estimated there are a total of 5 nonillion microorganisms on earth (5x1020). You will find nearly 10 million a millilitre of ocean water and 40 million in a gram of soil. They are also ubiquitous on the human body.
Microorganisms can be, according to their characteristics, divided into several groups: 1  bacteria, viruses, certain fungi and parasites. 2  pathogenic (capable of causing disease), non-pathogenic, and opportunistic (causing disease when they have an opportunity, like in people with low immune system).icroorganisms, like certain bacteria and yeasts, living on the human skin or in the nose, mouth, throat, small and large intestine and vagina, are part of the normal human flora; they prevent overgrowth of harmful microorganisms. Some of these microbes, when overgrow, may become pathogenic, though.