this association the organism of low
virulence will frequently acquire a higher degree of virulence, which
may be still further raised by means of "passages" (_vide supra_).

3. _Virus Plus Toxins._--The inoculation of pure cultivations of the
organism into some selected situation, together with the subcutaneous,
intraperitoneal, or intravenous injection of a toxin--e. g., one of
those elaborated by the proteus group--either simultaneously with,
before, or immediately after, the injection of the feeble virus. By
this means the natural resistance of the animal is lowered, and the
organism inoculated is enabled to multiply and produce its pathogenic
effect, its virulence being subsequently exalted by means of "passages."

~Attenuating the Virulence of an Organism.~--Attenuating or lowering the
virulence of a pathogenic microbe is usually attained with much less
difficulty than the exaltation of its virulence, and is generally
effected by varying the environment of the cultivations, as for example:

1. Cultivating in such media as are unsuitable by reason of their (a)
composition or (b) reaction.

2. Cultivating in suitable media, but at an unsuitable temperature.

3. Cultivating in suitable media, but in an unsuitable atmosphere.

4. Cultivation in suitable media, but under unfavorable conditions as to
light, motion, etc.

Attenuation of the virus can also be secured by

5. Passage through naturally resistant animals.

6. Exposure to desiccation.

7. Exposure to gaseous disinfectants.

8. By a combination of two or more of the above methods.

IMMUNISATION.

The further study of the pathogenetic powers of any particular bacterium
involves the active immunisation of one or more previously normal
animals. This end may be attained by various means; but it must be
remembered that immunisation is not carried out by any hard and fast
rule or by one method alone, but usually by a combination of methods
adapted to the exigencies of each particular case. The ordinary methods
include:

A. Active Immunisation.

I. By inoculation with dead bacteria (i. e., bacteria
killed by heat; the action of ultra-violet rays, of chemical
germicides, or by autolysis).

II. By the inoculation of attenuated strains of bacteria.

III. By the inoculation of living virulent bacteria (exalted
in virulence if necessary).

B. Combined Active and Passive Immunisation:

IV. By the inocula