=Parasitic Bacteria= live in other living organisms; but the relation between “host” and parasite may vary in considerable degree. Some parasites do no injury to their host, others produce dangerous contagious diseases; some choose only a special kind as host, others again live equally well in many different ones. There are further specific and individual differences with regard to the _predisposition_ of the host, and every individual has not the same receptivity at all times. THE HARMLESS PARASITES OF HUMAN BEINGS. Several of the above mentioned saprophytes may also occur in the alimentary canal of human beings; _e.g._, the Hay-bacillus, the Butyric-acid-bacillus, etc.; but the gastric juice prevents the development of others, at all events in their vegetative condition. _Sarcina ventriculi_, “packet-bacterium,” is only known to occur in the stomach and intestines of human beings, and makes its appearance in certain diseases of the stomach (dilation of the stomach, etc.) in great numbers, without, however, being the cause of the disease. It occurs in somewhat cubical masses of roundish cells (Fig. 25). LESS DANGEROUS PARASITES. In the mouth, especially between and on the teeth, a great many Bacteria are to be found (more than fifty species are known), _e.g. Leptothrix buccalis_ (long, brittle, very thin filaments which are united into bundles), Micrococci in large lumps, _Spirochæte cohnii_, etc. Some of them are known to be injurious, as they contribute in various ways to the decay of the teeth (_caries dentium_); a _Micrococcus_, for instance, forms lactic acid in materials containing sugar and starch, and the acid dissolves the lime salts in the external layers of the teeth: those parts of the teeth thus deprived of lime are attacked by other Bacteria, and become dissolved. Inflammation in the tissues at the root of a tooth, is probably produced by septic materials which have been formed by Bacteria in the root-canal. DANGEROUS PARASITES. In a large number of the infectious diseases of human beings and animals, it has been possible to prove that parasitic Bacteria have been the cause of the disease. Various pathogenic Bacteria of this nature, belonging to the coccus, rod, and spiral Bacteria groups, are mentioned in the following:-- =Pathogenic Micrococci.= _Staphylococcus pyogenes aureus_ produces abscesses of various natures (boils, suppurative processes in internal organs). The same effects are produced by-- _Streptococcus pyogenes_, which is the most frequent cause of malignant puerperal fever; it is perhaps identical with-- _Streptococcus erysipelatis_, which is the cause of erysipelas in human beings. _Diplococcus pneumoniæ_ (A. Fränkel) is the cause of pneumonia, and of the epidemic cerebro-spinal meningitis. _Gonococcus_ (Neisser) is the cause of gonorrhea and inflammation of the eyes.