general bacteriology 
53 
(which is a skin invader) be swallowed, no harm 
would, in all probability, result. 
2. Bacteria should invade the body in sufficiently 
large numbers. 
3. They should find an environment which is favor¬ 
able to their nutritional requirements. 
4. The resistance of the individual attacked should 
be sufficiently weakened to permit the bacteria to de¬ 
velop in his body. (This is discussed fully in a subse¬ 
quent section on Immunity.) 
5. We have mentioned in a preceding chapter that 
all bacteria are divided into two classes, according to 
their activities, the saprophytes, those which feed on 
dead organic matter and are highly useful in maintain¬ 
ing the chemical balance between the animal and the 
vegetable kingdoms, and the parasites or pathogenic 
(disease-producing) bacteria which feed on the liv¬ 
ing body and produce disease; although this division 
is not absolute, since many saprophytes may become 
under special conditions pathogenic (for example, they 
may gain entrance to a gangrenous limb and while 
feeding on dead organic matter will, nevertheless, 
cause disease by producing poisonous substances), yet 
it is convenient to separate the bacteria into these two 
classes; it is evident that in order that the infection 
may take place, the bacteria must be pathogenic. 
6. Even the pathogenic bacteria differ very much 
within the same species as to the degree of their power 
to incite disease; such power is spoken of as virulence; 
for example, pneumococci kept artificially (that is, in 
culture media) are much less virulent than those freshly 
isolated from the animal tissues. 
For a successful infection it is necessary, therefore, 
that the pathogenic bacteria shall possess sufficient 
virulence. 
