Bacteria are one-celled germs that live in the infected host outside of the body cells. Bacteria are large enough to see with a light microscope, as opposed to to viruses, which are many times smaller than bacteria and thus invisible with ordinary microscopes. Bacteria cause diseases in which are generally combatted in the body by white cells known as neutrophils, and thus are associated with the formation of pus.

Bacteria are classified into two main groups, based on their color when stained with a special stain and examined under the microscope. They are said to be either "Gram-positive" or "Gram-negative" based on this stain.

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