Source : Webster's Revised Unabridged Dictionary (1913)

Germ theory \Germ theory\
   1. (Biol.) The theory that living organisms can be produced
      only by the development of living germs. Cf. {Biogenesis},
      {Abiogenesis}.

   2. (Med.) The theory which attributes contagious and
      infectious diseases, suppurative lesions, etc., to the
      agency of germs. The science of bacteriology was developed
      after this theory had been established.

Germ \Germ\, n. [F. germe, fr. L. germen, germinis, sprout, but,
   germ. Cf. {Germen}, {Germane}.]
   1. (Biol.) That which is to develop a new individual; as, the
      germ of a fetus, of a plant or flower, and the like; the
      earliest form under which an organism appears.

            In the entire process in which a new being
            originates . . . two distinct classes of action
            participate; namely, the act of generation by which
            the germ is produced; and the act of development, by
            which that germ is evolved into the complete
            organism.                             --Carpenter.

   2. That from which anything springs; origin; first principle;
      as, the germ of civil liberty.

   {Disease germ} (Biol.), a name applied to certain tiny
      bacterial organisms or their spores, such as Anthrax
      bacillus and the {Micrococcus} of fowl cholera, which have
      been demonstrated to be the cause of certain diseases. See
      {Germ theory} (below).

   {Germ cell} (Biol.), the germ, egg, spore, or cell from which
      the plant or animal arises. At one time a part of the body
      of the parent, it finally becomes detached,and by a
      process of multiplication and growth gives rise to a mass
      of cells, which ultimately form a new individual like the
      parent. See {Ovum}.

   {Germ gland}. (Anat.) See {Gonad}.

   {Germ stock} (Zo["o]l.), a special process on which buds are
      developed in certain animals. See {Doliolum}.

   {Germ theory} (Biol.), the theory that living organisms can
      be produced only by the evolution or development of living
      germs or seeds. See {Biogenesis}, and {Abiogenesis}. As
      applied to the origin of disease, the theory claims that
      the zymotic diseases are due to the rapid development and
      multiplication of various bacteria, the germs or spores of
      which are either contained in the organism itself, or
      transferred through the air or water. See {Fermentation
      theory}.

Source : WordNet®

germ theory
     n : (medicine) the theory that all contagious diseases are
         caused by microorganisms