There is evidence that a mixed infection—­that is, the introduction of more than one species of organism, for example, the tubercle bacillus and a pyogenic staphylococcus—­increases the severity of the resulting disease.  If one of the varieties gain the ascendancy, the poisons produced by the others so devitalise the tissue cells, and diminish their power of resistance, that the virulence of the most active organisms is increased.  On the other hand, there is reason to believe that the products of certain organisms antagonise one another—­for example, an attack of erysipelas may effect the cure of a patch of tuberculous lupus.

Lastly, in patients suffering from chronic wasting diseases, bacteria may invade the internal organs by the blood-stream in enormous numbers and with great rapidity, during the period of extreme debility which shortly precedes death.  The discovery of such collections of organisms on post-mortem examination may lead to erroneous conclusions being drawn as to the cause of death.

#Results of Bacterial Growth.#—­Some organisms, such as those of tetanus and erysipelas, and certain of the pyogenic bacteria, show little tendency to pass far beyond the point at which they gain an entrance to the body.  Others, on the contrary—­for example, the tubercle bacillus and the organism of acute osteomyelitis—­although frequently remaining localised at the seat of inoculation, tend to pass to distant parts, lodging in the capillaries of joints, bones, kidney, or lungs, and there producing their deleterious effects.

In the human subject, multiplication in the blood-stream does not occur to any great extent.  In some general acute pyogenic infections, such as osteomyelitis, cellulitis, etc., pure cultures of staphylococci or of streptococci may be obtained from the blood.  In pneumococcal and typhoid infections, also, the organisms may be found in the blood.

It is by the vital changes they bring about in the parts where they settle that micro-organisms disturb the health of the patient.  In deriving nourishment from the complex organic compounds in which they nourish, the organisms evolve, probably by means of a ferment, certain chemical products of unknown composition, but probably colloidal in nature, and known as toxins.  When these poisons are absorbed into the general circulation they give rise to certain groups of symptoms—­such as rise of temperature, associated circulatory and respiratory derangements, interference with the gastro-intestinal functions and also with those of the nervous system—­which go to make up the condition known as blood-poisoning, toxaemia, or bacterial intoxication.  In addition to this, certain bacteria produce toxins that give rise to definite and distinct groups of symptoms—­such as the convulsions of tetanus, or the paralyses that follow diphtheria.