Q ETIOLOGY 
cause of an infectious disease we look for some species of micro- 
organism which may belong either to the animal or to the vegetable 
kingdom. The fact that certain microscopic animals and plants 
have become, if they were not in the beginning, parasitic on larger 
and higher forms of life has long been recognized; but the idea 
came later, that the various infections giving rise to a wide series 
of phenomena, known as symptoms and morbid anatomy, were 
the direct results of the invasion of the individual with living micro- 
scopic plants (bacteria) or animals (protozoa). It is likewise true 
that for many general disorders the cause may be found in the condi- 
tions of life under which the individual has been forced to exist. 
Etiology. therefore, in a broad sense, includes both the infecting 
microorganisms that cause the specific infectious diseases and poor 
hygiene, insanitary conditions and physical forces that may produce 
non-specific morbid changes often sufficient to cause death. 
Infection. The term infection is generally understood to mean 
the entrance into the animal body, from without, of living micro- 
organisms capable of multiplying within the living tissues and of 
producing in consequence of this multiplication a local or a general 
diseased condition and perhaps the death of the individual. The 
invading microorganisms may belong to any one of three groups of 
microscopic life, namely, bacteria, higher fungi, and protozoa. 
It is customary and convenient, if not altogether logical, to limit 
the term microorganisms to these forms, excluding altogether the 
entozoa and other animal parasites, most of which are not micro- 
scopic in size. 
Intoxication. A diseased condition produced by substances not 
capable of reproducing themselves, as, for example, organic or inor- 
ganic chemical compounds, is an intoxicative process. In an infec- 
tion, the immediate cause of the symptoms and morbid changes 
in the tissues affected may be an intoxication due to the action of 
the metabolic products (toxins) of the invading microérganisms. 
The theories of the mechanical interference of the invading organ- 
isms with the normal functions of the body or that they absorb 
the nutriment, thus depriving the tissues of necessary food, wait for 
demonstration. The results of infection varv in their manifestations. 
Wound Infection. If the invading organisms remain at the point 
of entrance and produce local tissue changes, the condition is spoken 
of as a wound infection. 
