Bacteria: Single-celled microorganisms which can exist either as independent (free-living) organisms or as parasites (dependent upon another organism for life).

Examples of bacteria include: Acidophilus, a normal inhabitant of yogurt, Chlamydia, which causes an infection very similar to gonorrhea, Clostridium welchii the most common cause of the dreaded gas gangrene, E. coli, the common peaceful citizen of our colon and, upon occasion, a dangerous agent of disease, and Streptococcus, the bacterium that causes the important infection of the throat strep throat.

The term bacteria was devised in the 19th century by the German botanist Ferdinand Cohn (1828-98) who based it on the Greek bakterion meaning a small rod or staff. In 1853, Cohn categorised bacteria as one of three types of microorganisms -- bacteria (short rods), bacilli (longer rods), and spirilla (spiral forms). The term bacteria was preceded in the 17th century by the microscopic animalcules described by Antony van Leeuwenhoek (1632-1723).