_Diphtheria._—A disease of human beings chiefly. It is caused by a bacillus which was described in 1883–84 by Klebs and Loeffler, and is known as _Bacillus diphtheriae_, or Klebs-Loeffler bacillus. Its mode of entry is by inhaling infected air, or by drinking or eating infected milk or food. _Cholera._—This disease is peculiar to human beings. Its native home is on the banks of the river Ganges in India, where Koch in 1884 was able to isolate its causative spirillum. Man is infected by drinking contaminated water or by contact. _Lockjaw, or Tetanus._—Afflicts man, horses, and dogs. The _Bacillus tetani_ is the most deadly of all known bacteria. It enters the body by wounds. It was discovered in 1884 by Nicolaier. _Influenza, or the Grip._—Caused by one of the smallest-known bacilli; discovered in 1892 by Canon and Pfeiffer. Infection spreads by the scattering about by air-currents of the dried nasal and bronchial secretion of those suffering from the disease, and its portal of entry is by the nose and bronchial tubes. _Pneumonia._—Caused by a coccus which grows in pairs and small chains. It enters the body by means of the respiratory tract. It is present in the saliva of twenty per cent. of healthy persons. Proved by Frankel in 1886 to be the cause of this disease. _Bubonic Plague._—In 1894 Kitasato and Yersin isolated a small bacillus in a large number of cases and proved it to be the cause. It enters the body by means of wounds of the skin, and through bites of fleas from infected rats, which are said to be one of the chief factors in spreading this dread malady. _Yellow Fever._—The cause of this disease is still under discussion. Such are a few of the infectious diseases which we can readily attribute to the presence of definite micro-organisms in respective cases. But strange as it may seem, the most typical of all infectious diseases, small-pox, scarlet fever, measles, and hydrophobia, have as yet not yielded up their secrets. This is possibly due to the minute size of the micro-organisms concerned, which make it beyond the power of the best microscope to demonstrate them. In this connection it has recently been shown by Roux and Nocard that in the case of the disease known as pleuro-pneumonia of cattle the causative agent is so very small as just to be barely visible. Again, it is quite possible that these diseases may be caused by living things we know nothing about, which may be quite dissimilar from the bacteria. INFECTION—ITS PROCESSES AND RESULTS In the foregoing list of diseases associated with specific bacteria, attention has been drawn to the common modes of infection, or, as they are technically called, “portals of entry,” and it now remains to touch upon the main factors, processes, and results following upon the entry into the body of such disease-producing microbes.