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.
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