even diffuse daylight, is inimical to the growth of bacteria, as are
also Röntgen rays and radium emanations.

#Pathogenic Properties of Bacteria.#--We are now only concerned with
pathogenic bacteria--that is, bacteria capable of producing disease in
the human subject. This capacity depends upon two sets of factors--(1)
certain features peculiar to the invading bacteria, and (2) others
peculiar to the host. Many bacteria have only the power of living upon
dead matter, and are known as _saphrophytes_. Such as do nourish in
living tissue are, by distinction, known as _parasites_. The power a
given parasitic micro-organism has of multiplying in the body and giving
rise to disease is spoken of as its _virulence_, and this varies not
only with different species, but in the same species 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