Microorganisms are minute organisms of microscopic dimensions, too small to be seen by the eye alone. To be viewed, microorganisms must be magnified by an optical or electron microscope. The most common types of microorganisms are viruses, bacteria, blue-green bacteria, some algae, some fungi, yeasts, and protozoans.

Viruses, bacteria, and blue-green bacteria are all prokaryotes, meaning that they do not have an organized cell nucleus separated from the protoplasm by a membrane-like envelope. Viruses are the simplest of the prokaryotic life forms. They are little more than simple genetic material, either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid), plus associated proteins of the viral shell (called a capsid) that together comprise an infectious agent of cells. Viruses are not capable of independent reproduction. They reproduce by penetrating a host cell and diverting much of its metabolic and reproductive physiology to the reproduction of copies of the virus.

The...