Quite as astonishing is the discovery that within the root-knobs of pease and beans live bacteria which by splitting up mineral salts containing nitrogen, and by absorbing nitrogen from the air, give it over to the plant so that it is enabled to grow luxuriantly, whereas, without their presence, the tiller of the soil might fertilize the ground in vain. It is quite possible that not alone pease and beans, but all grasses and plants and trees depend upon the presence of such germs for their very existence, which in turn supply man and animals with their means of existence. Hence we see that these nitrifying bacteria, as they are called, if swept out of existence, would be the cause of cessation of all life upon the globe. And arguing backward, one prominent authority states it as his belief that the first of all life on this earth were those lowly forms of plants which only required the nitrogen of air or salts to enable them to multiply. Limiting observation now to the sphere of medicine, it will be readily perceived that the presence of bacterial life in a causative relation to disease is an object of paramount regard. The following paragraphs will briefly treat of the diseases associated with micro-organisms and the common modes of infection in each, the chain of events subsequent to an infection, and the possibilities of protection or cure by means of substances elaborated in the body of an individual or animal recently recovered from an infectious disease: _Anthrax._—A disease chiefly of cattle and sheep, occasionally of man, is caused by the _Bacillus anthracis_, discovered in 1849–50 by Pollender and Davaine. It enters the body through abrasions of the skin, by inhalation of the spores, or seeds, into the lungs, or by swallowing infected material. _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.