by the organisms of diphtheria, pneumonia, or erysipelas is much
increased. Even such slight depression of vitality as results from
bodily fatigue, or exposure to cold and damp, may be sufficient to turn
the scale in the battle between the tissues and the bacteria. Age is an
important factor in regard to the action of certain bacteria. Young
subjects are attacked by diphtheria, tuberculosis, acute osteomyelitis,
and some other diseases with greater frequency and severity than those
of more advanced years.

In different races, localities, environment, and seasons, the pathogenic
powers of certain organisms, such as those of erysipelas, diphtheria,
and acute osteomyelitis, vary considerably.

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.