Microbes, also known as microorganisms, are defined solely on the basis of their size—they are too small to be seen by the naked eye. Microorganisms can only be viewed if they are magnified in size, using an optical or electron microscope.

Apart from their size, the major groups of microorganisms have little affinity in terms of their evolutionary history and systematics. Included among the microorganisms are viruses, bacteria, blue-green bacteria, some algae, some fungi, yeasts, and protozoans.

Viruses

Although viruses are commonly considered to be microorganisms, they are actually "pseudo-organisms" because they do not display all of the characteristics of life. Viruses consist only of bits of nucleic acid (either deoxyribonucleic acid (DNA), or ribonucleic acid (RNA)) surrounded by a protein capsule (called a capsid). Viruses are not capable of independent reproduction, and they cannot perform other important metabolic functions. To reproduce and grow viruses...