he nature of an
intoxication. There is, however, another all-important factor concerned,
viz. the multiplication of the living organisms in the tissues; this is
essential to, and regulates, the supply of toxins. It is important that
these two essential factors should be kept clearly in view, since the means
of defence against any disease may depend upon the power either of
neutralizing toxins or of killing the organisms producing them. It is to be
noted that there is no fixed relation between toxin production and
bacterial multiplication in the body, some of the organisms most active as
toxin producers having comparatively little power of invading the tissues.

[Sidenote: The production of disease.]

We shall now consider how bacteria may behave when they have gained
entrance to the body, what effects may be produced, and what circumstances
may modify the disease in any particular case. The extreme instance of
bacterial invasion is found in some of the septicaemias in the lower
animals, _e.g._ anthrax septicaemia in guinea-pigs, pneumococcus
septicaemia in rabbits. In such diseases the bacteria, when introduced into
the subcutaneous tissue, rapidly gain entrance to the blood stream and
multiply freely in it, and by means of their toxins cause symptoms of
general poisoning. A widespread toxic action is indicated by the lesions
found--cloudy swelling, which may be followed by fatty degeneration, in
internal organs, capillary haemorrhages, &c. In septicaemia in the human
subject, often due to streptococci, the process is similar, but the
organisms are found especially in the capillaries of the internal organs
and may not be detectable in the peripheral circulation during life. In
another class of diseases, the organisms first produce some well-marked
local lesion, from which secondary extension takes place by the lymph or
blood stream to other parts of the body, where corresponding lesions are
formed. In this way secondary abscesses, secondary tubercle glanders and
nodules, &c., result; in typhoid fever there is secondary invasion of the
mesenteric glands, and clumps of bacilli are also found in internal organs,
especially the spleen, though there may be little tissue change around
them. In all such cases there is seen a selective character in the
distribution of the lesions, some organs being in any disease much more
liable to infection than others. In still [v.03 p.0175] another class of
diseases the bacteria are restric