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There are millions of microbes, or microscopic organisms, most of which are too small to see with the naked eye. They live inside and around other living beings, including humans, in every kind of environment.
Only a minority of these microbes actually cause serious diseases. Many have no effect on humans, and some are life-sustaining, such as those that help us use vitamins or fight microbes that produce infection and disease.
There are three
major groups of microbes: viruses,
bacteria and protozoa.
Within each group are numerous species that differ in shape, size, and
structure, as well as food intake and reproductive processes. Some larger
organisms, such as flukes and worms,
and fungi, can also cause infectious
diseases.
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