INFECTION. 103 



virulent form and yet escape the disease. On the other hand, 

 certain individuals will succumb rapidly even when the infec- 

 tive agent is attenuated. Again, the severity of the disease 

 depends not only on the individual susceptibility, but also 

 upon the vitality of the germ, the source of the infection, and 

 the time and point of entrance of the infective agent. 



The accidental presence of a germ in the body of an indi- 

 vidual during disease cannot be accepted as evidence of its 

 etiologic relationship with that particular disease. Koch has 

 laid down some requirements, known as Koch's law, with 

 which the organism must comply before it can be considered 

 as the specific cause of a disease : 



1. That the organism should always be present in the 



body of the animal having the disease, and that 

 its presence must explain the changes met with in 

 that disease. 



2. That it must be possible to isolate and make a pure 



culture of the organism outside of the body. 



3. That the inoculation of an animal with a pure culture 



of this organism will result in the production of the 

 typical disease from which the germ was obtained. 



4. That the typical organism be found in the tissues of 



the animal thus inoculated. 



A number of organisms have been accepted as specific that 

 do not meet all of these requirements, but the evidence in 

 their favor is so overwhelming that the specificity cannot be 

 doubted. The tubercle bacillus is an example. It has not, 

 according to many observers, as yet met the last two require- 

 ments in man. 



Bacteria are phlogistic? producing simply a local inflamma- 

 tory reaction (staphylococcus) ; toxic, a local growth (/. e., 

 without invading the body themselves), with rapid and exten- 

 sive dissemination of the toxin (tetanus and diphtheria) ; 

 septic, invading (themselves) the body-fluids (anthrax). 



A disease is said to be sporadic when only isolated cases 

 appear ; endemic, when the disease is always present ; epidemic, 

 when it is unusually prevalent and exhibiting a marked ten- 

 dency to spread beyond its usual limitations ; pandemic, when 

 the disease is widely spread, as in several states or countries. 



