=Bacteria= are exceedingly minute specks of naked protoplasm. They are unicellular plants. Some of them are harmless to mankind; some are very useful to him, and others are his deadly enemies. Many of them are concerned in the production of the infectious diseases. They do so by elaborating various chemical products that are virulent poisons, hence these products are called toxines; when taken up by the blood, they are carried to various parts of the body. In this manner they cause the particular symptoms that are characteristic of a special infectious disease. Why is it that some persons, on exposure to an infectious disease, contract the malady while others similarly exposed do not? In other words, what gives immunity to disease? The explanation is probably as follows: Just as the invading bacteria have the power of secreting _toxines_, so the cells of the body, normally, have the power of elaborating chemical products that are antidotes to the toxines, and are appropriately called _antitoxines_. Infectious diseases and immunity from them, are the result of a contest between the invading bacteria and the protecting cells of the body. If the bacteria secrete toxines in greater quantities than can be neutralized by the cells of the body, we have disease; if the reverse occur, we have immunity. The =white blood-corpuscles= (cells) also take part in this warfare. They have the power of traveling, in virtue of their amœboid movements, from the blood to the part invaded by the bacteria. Here a contest takes place between them, the corpuscle takes the bacteria into its interior, and either kills them or is itself killed. The result of this contest helps to produce either immunity or infection. =Tetanus bacillus= is a cell shaped like a slender rod. It has the power of secreting a poison which, when introduced into the body, produces convulsions and other symptoms of lockjaw. These much resemble those induced by strychnine poisoning. =Bacillus diphtheriæ= is an exceedingly small unicellular plant, and has the power of producing a poison called _toxalbumin_, which is analogous to the poison of certain venomous serpents. It is the speck of protoplasm through whose activity diphtheria is caused.