Parasitic Diseases constitute an important sub-group in the accepted classification of Disease (q.v.). In these diseases certain morbid conditions are induced by the presence of animals or vegetables which have found a place of subsistence within some tissue or organ, or upon some surface of the body of man or other animals. Plants are not exempt from disorders of this nature (see PARASITIC PLANTS). The forms of animal life giving rise to parasitic diseases are described in articles on Ascaris, Cestoid Worms, Flea, Guinea-worm, Itch, Lice, Nemathelminia, Strongylus, Tape-worms, Thread-worms, Tick, Trichina, &c.
The vegetable organisms which are associated with special diseases are almost all of microscopic size, and therefore, though their effects are of much greater importance than those of animal parasites, they are as yet much less perfectly understood. Certain minute fungi have long been recognised as the causes of diseases in the skin and mucous membranes: Favus, Pityriasis versicolor, Ring-worm, Thrush (q.v.). It was shown in 1861 by Carter that a serious disease of the foot which occurs in India (Madura-foot, fungus-foot, &c.) is due to the presence of a fungus; and in 1877 what is now called Actinomycosis (q.v.) was put in the same category.
But the most important and interesting of the vegetable parasites are those belonging to the Schizomycetes or Bacteria (q.v.), whose study has assumed such prominence that it is now almost an independent science (Bacteriology). The relations of these organisms to their host are much more intimate than in the case of the larger parasites, and the problems presented by the diseases associated with them are consequently much more difficult of solution; but in some cases the parasitic nature of these diseases has been completely established. Analogy makes it probable that some day all 'specific febrile diseases' will have to be included in this group. See GERM THEORY.