ies at different times
and under varying circumstances. The actual number of organisms
introduced is also an important factor in determining their pathogenic
power. Healthy tissues can resist the invasion of a certain number of
bacteria of a given species, but when that number is exceeded, the
organisms get the upper hand and disease results. When the organisms
gain access directly to the blood-stream, as a rule they produce their
effects more certainly and with greater intensity than when they are
introduced into the tissues.

Further, the virulence of an organism is modified by the condition of
the patient into whose tissues it is introduced. So long as a person is
in good health, the tissues are able to resist the attacks of moderate
numbers of most bacteria. Any lowering of the vitality of the
individual, however, either locally or generally, at once renders him
more susceptible to infection. Thus bruised or torn tissue is much more
liable to infection with pus-producing organisms than tissues clean-cut
with a knife; also, after certain diseases, the liability to infection
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 sufferi