Virus , a Latin word for "poison," is the technical name of a large class or protein substances so small that only the electron microscope can make them visible. Capable of existing indefinitely in the form of crystals, they are seemingly lifeless until contact with the appropriate cells of a living organism triggers them into a reproductive activity whose power, subtlety and virulence have given us such infective and frequently epidemic diseases as polio, smallpox, influenza, rabies and the still unpreventable common cold.

The general term for all typically one-celled forms of life is micro-organism , a word that includes both plants and animals. Microbe was popularized by Pasteur, who applied it to all forms of micro-organisms , replacing the much older word animalcule , restricted to microscopic animals and now only rarely used. Because Pasteur’s work was so largely concerned with harmful varieties, microbe came to suggest only those disease-producing organisms of which the correct scientific name is disease-producing organisms of which the correct scientific name is pathogen . In this sense it is still colloquially used: microbe hunters; the microbe of tuberculosis.

Germ , as in the expression "germ theory of disease" made popular by the work of Koch and others, acquired a similar meaning, though strictly it refers to the living part of a seed or to any minute, not necessarily harmful, organism: a wheat germ ; germ cell; germ plasm.

Bacterium (the singular of the better-known plural bacteria ) is the inclusive term for any of numerous, widely distributed micro-organisms , exhibiting both plant and animal characteristics. Though most of them are harmless to man, and many are essentially to the processes of life, some are pathogens that cause serious, often fatal, diseases, such as cholera, diphtheria, tetanus, syphilis and botulism. Bacillus , meaning "a little rod," denotes a member of one of the three broad classes of bacteria identified by characteristic shapes and modes of action.