Leprosy.—This disease is caused by a bacillus known as Bacillus leprae, which was discovered by Hansen in 1879. It is doubtful if it has been grown outside the body. It is supposed to enter by abrasions of the skin, but it is very feebly contagious, notwithstanding popular ideas as to its supposedly highly contagious nature.
Tuberculosis.—All forms of this disease, among which is ordinary consumption, are caused by a bacillus closely resembling that of leprosy. It was discovered by Koch in 1880–82, and named Bacillus tuberculosis. The ways of infection are by inhaling the dried sputum of consumptives, drinking infected cow’s milk, or eating infected meat.
Typhoid Fever.—A disease of human beings only. Eberth in 1880 discovered the germ causing it and called it Bacillus typhosus. It gains entrance to our bodies chiefly in the milk and water we drink, which comes from infected sources; a rarer method is by inhalation of infected air.
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.