82 INFECTION 



cocci in the tonsils of most persons, usually harmless, but capable, under 

 special conditions, of producing severe and even fatal infection. 



The abnormal state resulting from the deleterious local and general 

 interaction between a host and an invading bacterium, with consequent 

 tissue changes and symptoms, constitutes an infectious disease. 



As has previously been stated, not every invasion of the deeper 

 tissues by microorganisms results in injury or disease. A certain num- 

 ber of bacteria are constantly gaining admission to the deeper tissues 

 of the alimentary and the respiratory tracts, without producing apparent 

 injury to the host, as they tend to be destroyed very soon after they 

 gain entrance. The terms invasion and infection are not, therefore, 

 synonymous. Every true infection is accompanied by local changes, 

 although these may be so slight as to escape notice; an infectious disease 

 is practically made up of similar phenomena, but these are of an ex- 

 aggerated or marked degree. 



The hygienist distinguishes between (1) Sporadic, or isolated, cases 

 of infection; (2) endemic, in which a certain microbic disease affects the 

 inhabitants of a given area year after year; and (3) epidemic, in which a 

 disease appears suddenly and affects a large number of inhabitants, the 

 number of cases rapidly increasing and decreasing. Among the lower 

 animals equivalent terms for the types just described are sporadic, 

 enzootic, and epizootic. A pandemic disease is one that is epidemic over 

 a large territory. 



In all infections there are two inseparable factors to be considered : 



1. The offensive forces of the infecting agent, dependent upon its 

 pathogenic or disease-producing nature and its power of defending itself 

 against the antagonistic forces of the host and of thriving under these 

 conditions. 



2. The resistance offered by the host, and mainly dependent upon 

 certain physical or non-specific local factors or specific antibodies, which 

 constitute the defensive mechanism, or immunologic factors. 



The former is concerned with the general subject of infection, and 

 the latter with that of immunity. 



Microorganisms and host may live together in apparent harmony, 

 owing to the ability of the host to restrain the activity of the micro- 

 organism and neutralize its injurious effects or to an absence of infec- 

 tivity on the part of the microorganism until the vital resistance of the 

 host is diminished or the pathogenicity of the microorganism is increased, 

 when the neutral relations are disturbed and infection occurs. 



