56 BACTERIOLOGY 



eased animals, especially tuberculous cattle, suggest themselves 

 as obvious precautions; whilst the reduction of germ-laden 

 dust on our roads and in our cities is a reform urgently re- 

 quired ; such dust acting, not only as a carrier of disease, but 

 by its entrance into the lungs damaging the tissues and pre- 

 paring the way for the action of the accompanying bacteria. 



For the methods by which organisms spread and cause dis- 

 ease within the body, reference must be made to larger works. 

 It is sufficient here to mention that some bacteria, such as the 

 diphtheria bacillus, usually remain localised in the throat or 

 neighbouring parts, their toxins alone being carried into the 

 tissues of the body by the lymph- and blood-streams. Similarly, 

 tetanus bacilli remain localised in some wound, often quite 

 small, and manufacture their toxins, which in the same way 

 pass in and poison the body tissues. Again, some organisms, 

 e.g. the cholera germ, remain localised in the intestine, whilst 

 others, such as the typhoid bacilli, are carried far and wide 

 throughout the body. Some organisms which are usually de- 

 scribed as causing wound infection may produce a trivial local 

 inflammation, serious spreading inflammatory conditions such as 

 erysipelas, abscesses and the like, or general infection of the 

 whole body, as in septicaemias and pyaemias. The severity of 

 the disease depends on the virulence, number, method and 

 site of introduction of the causal organism, the presence of 

 other bacteria, the relative susceptibility or otherwise of the 

 tissues and also the presence of other diseased conditions in 

 the body, as indicated in our chapter on Immunity. 



Before passing to the consideration of the organisms which 

 produce disease, the following terms may be briefly defined. 



Inflammation : a very indefinite term, which embraces the 

 " series of reactions in the tissues which follows irritation or 

 injury, and which has for its special purpose the prevention or 

 repair of damage/' bacteria and their toxins being the common- 

 est and most important causal agents. Inflammation may be 

 acute, subacute, or chronic, according to the time it lasts. 



Suppuration : is a form of inflammation usually caused by 

 organisms which possess the property of softening and digest- 

 ing the tissue by means of digestive ferments, the pus or 

 " matter " so formed consisting of leucocytes which pass in to 

 attack the organisms, the softened and liquefied tissues of the 

 part, and the bacteria themselves and their toxins. Suppura- 

 tion may occur on a surface as a purulent catarrhal inflam- 

 mation, or may be localised in the substance of a tissue as an 

 abscess. It may spread through the tissues as a purulent infil- 

 tration, e.g. in erysipelas, or it may occur in one of the serous 

 cavities of the body such as the peritoneal lining of the abdomen, 



