refers to a unicellular, prokaryotic microorganism. This is an organism whose chromosome is not help in a nuclear envelope. Bacteria are considered to represent an earlier stage in the evolution of life than eukaryotes, (organisms whose chromosomes are contained in nuclear envelopes, i.e., all organisms except bacteria). In humans, many different types of bacteria cause disease (e.g., tetanus, syphilis, tuberculosis), while others are beneficial, in that they may increase the body's defenses against microorganisms.
BACTERIUM: "A bacterium is the singular name for bacteria, a unicellar microorganism that in general, causes sickness in the carrier."