INFECTION AND IMMUNITY. 
31 
infection depends in part on their power of producing 
disease, that is, their virulence. Conditions that are 
not suited to the growth of bacteria will diminish or 
destroy the virulence; the continued cultivation of bac¬ 
teria outside the body on artificial culture media will 
do this. Bacteria that have lost the power of pro¬ 
ducing disease are spoken of as being attenuated. 
Another factor that modifies infection is the number 
of bacteria that invade the tissues. While the exact 
number of bacteria necessary to cause infection is not 
known, it may be said that the greater the virulence 
the fewer the bacteria required. The path by which 
bacteria enter the tissues frequently determines 
whether infection is caused or not. The bacilli of 
typhoid fever to cause infection must be swallowed, 
but if they are rubbed into the skin no infection results. 
On the other hand, the pus-forming bacteria like the 
staphylococci and streptococci may be swallowed with¬ 
out causing infection, but if they are rubbed into the 
skin a boil or an abscess is almost sure to result. So 
to cause infection bacteria must enter the body through 
channels best adapted to their growth and multi¬ 
plication. 
Concerning the individual exposed to infection it 
is known that everyone is endowed to a variable de¬ 
gree with defensive substances in the blood and 
tissues that tend to overcome and destroy invading 
bacteria. Unhealthy people, as everyone knows, are 
more likely to become infected and to succumb to in¬ 
fection than the healthy. This power of the human 
On the 
part of 
bacteria 
Viru¬ 
lence 
Attenua¬ 
tion 
Avenue 
of infec¬ 
tion 
On the 
part of 
the indi¬ 
vidual 
