A micro-organism is a _parasite_ when it can live in animal tissues. It is a _saphrophyte_ when it can exist outside animal tissues. If a parasite cannot exist outside animal tissues, it is an _obligatory parasite_; if it can, it is a _facultative saphrophyte_. Similarly the saphrophytes are classed as obligatory saphrophytes and facultative parasites. Pathological micro-organisms have very complicated products which are in large part poisonous. Bacteriologists require seven conditions to prove a micro-organism the _specific_ cause of a given disease, and all these conditions have been fulfilled for anthrax, diphtheria, and tetanus. The specificity has been satisfactorily settled for glanders, malaria, tuberculosis, actinomycosis, gonorrhoea, and malignant oedema. It has been practically settled for typhoid, influenza, the Madura disease, and the bubonic plague; and incompletely defined for leprosy, relapsing fever, and Malta fever. There are certain diseases which are not called specific, because they may be produced by various micro-organisms. These are pneumonia, osteomyelitis, septicaemia, pymaeia, endocarditis, meningitis, erysipelas, angina Ludovici, broncho-pneumonia, and similar maladies. Cholera and dysentery also might be grouped with these, as cholera appears to be produced by various vibrios and dysentery by different amoebae. There are other infective diseases, in which we have not yet found the causative micro-organism, but we presume its existence. These are: rabies, syphilis, yellow fever, dengue, typhus, mumps, whooping-cough, smallpox, measles, scarlet fever, and others among the exanthemata. Malaria and similar diseases are caused by plasmodia, which are protozoa and not bacteria. {170} The priest is almost as frequently exposed to the danger arising from contagion as the physician is, and a priest that often ministers to the sick is liable to grow imprudently indifferent to danger. For one priest that is too much afraid of disease we find a hundred that have not sufficient dread.