【Germ】 , 【microbe】 , 【bacterium】 , 【bacillus】 , 【virus】 though not strict synonyms, are comparable because all denote organisms invisible to the naked eye, including organisms that are the causative agents of various diseases.
【Germ】 and 【microbe】 are the ordinary nonscientific names for such an organism and especially for one that causes disease. Bacteria, the plural of 【bacterium】 and the form commonly in general use, is often employed as the equivalent of germs and microbes.
Technically, it is the scientific designation of a large group of prokaryotic microbes which are found widely distributed in water, air, soil, living things, and dead organic matter, which have structural and biological characteristics distinguishing them from other unicellular microorganisms (as protozoans), and only some of which are instrumental in producing disease in man, animals, and plants.
In addition to the pathogenic or disease-causing bacteria there are the saprophytic bacteria which live upon dead or decaying organic matter and which, for the most part, are beneficial in their effects which include many natural chemical processes (as fermentation, oxidation, and nitrification).
【Bacillus】 is often employed as though it designated any of the pathogenic bacteria. In technical scientific usage it denotes any of a genus of bacteria which originally included all or most rod-shaped forms and is now restricted to a group of mostly soil-inhabiting, aerobic, and saprophytic forms that produce endospores.
However, it is often used of rod-shaped bacteria in general, especially as distinguished from those which are globe-shaped (the coccus form, of which the streptococcus is an example) and those which are spiral (the spirillum form, of which the vibrio which causes Asiatic cholera is an example, and the spirochete form, exemplified by the treponema of syphilis). It is common, especially in medical usage, to speak of the bacilli of such diseases as typhoid, diphtheria, and tetanus, though none of these are true bacilli in the restricted taxonomic sense.
【Virus】 , in earlier use, was an imperceptible infectious principle of unknown nature occurring in the body of a diseased individual and held to be involved in the transfer of infectious diseases. In this sense it has been applied to most germs or microbes while their specific nature remained unknown, as well as to bodily fluids and discharges containing such infective agents.
A vestige of this meaning persists in immunologic usage with respect to materials (as vaccine lymph) that are antigenic but not usually infective.
In general modern usage 【virus】 is equivalent to filterable 【virus】 and is restricted to a variety of parasitic and infective agents which are able in nature to multiply only in living tissues, are so small that they pass through the pores of bacteriological filters, and are generally invisible with the ordinary light microscope.
They include noncellular microbes (as herpesviruses, poliovirus, and tobacco mosaic 【virus】 ) that lie on the border between the living and nonliving, may consist of a single macromolecule of DNA or RNA in a protein case, and are capable on the one hand of existing in the crystalline state and on the other, when introduced into suitable cells, of multiplying like a true organism.
