may have varying powers of producing disease at different times.  Some species are universal inhabitants of the alimentary canal and are ordinarily harmless, while under other conditions of unknown character they invade the tissues and give rise to a serious and perhaps fatal disease.  We may thus recognise some bacteria which may be compared to foreign invaders, while others are domestic enemies.  The former, like the typhoid bacillus, always produce trouble when they succeed in entering the body and finding a foothold.  The latter, like the normal intestinal bacilli, are always present but commonly harmless, only under special conditions becoming troublesome.  All this shows that there are other factors in determining the course of a disease, or even the existence of a disease, than the simple presence of a peculiar species of pathogenic bacterium.

From the facts just stated it will be evident that any list of germ diseases will be rather uncertain.  Still, the studies of the last twenty years or more have disclosed some definite relations of bacteria and disease, and a list of the diseases more or less definitely associated with distinct species of bacteria is of interest.  Such a list, including only well-known diseases, is as follows: 

Name of disease.               Name of bacterium producing the disease. 
Anthrax (Malignant pustule).      Bacillus anthracis. 
Cholera.                          Spirillum cholera:  asiaticae
Croupous pneumonia.               Micrococcus pneumonia crouposa. 
Diphtheria.                       Bacillus diphtheria. 
Glanders.                         Bacillus mallei. 
Gonorrhoea.                       Micrococcus gonorrhaeae
Influenza.                        Bacillus of influenza. 
Leprosy.                          Bacillus leprae. 
Relapsing fever.                  Spirillum Obermeieri. 
Tetanus (lockjaw).                Bacillus tetani. 
Tuberculosis (including
consumption, scrofula, etc.)     Bacillus tuberculosis. 
Typhoid fever.                    Bacillus typhi abdominalis.

Various wound infections, including septicaemia, pyaemia, acute abscesses, ulcers, erysipelas, etc., are produced by a few forms of micrococci, resembling each other in many points but differing slightly.  They are found almost indiscriminately in any of these wound infections, and none of them appears to have any definite relation to any special form of disease unless it be the micrococcus of erysipelas.  The common pus micrococci are grouped under three species, Staphylococcus pyogenes aureus, Staphylococcus pyogenes, and Streptococcus pyogenes.  These three are the most common, but others are occasionally found.

In addition to these, which may be regarded as demonstrated, the following diseases are with more or less certainty regarded as caused by distinct specific bacteria:  Bronchitis, endocarditis, measles, whooping-cough, peritonitis, pneumonia, syphilis.