Infectious diseases are supposed to be propagated by the agency of minute living parasites given off from the body of the sick and conveying the specific virus. The germ thereby includes this for the cause of all the zymotic diseases—diseases that are contagious and produced by some morbid principle or germs acting on the system like a ferment. They are claimed to be a vegetable growth of a fungoid nature, and the theory is that during the process or period of each—a period of growth like mildew—the victim is a sufferer from a more or less violent fever; that the period varies; in typhoid it is twenty-one days, in other forms of fever perhaps a shorter time, till the microscopic fungoid growth may be said to effloresce and shed its spores. Some diseases also that are not contagious are believed to be caused by organisms in the air. There are various kinds; monads, bacteria, vibriones, &c., are among the substances found in the atmosphere of a large city, and elsewhere. A very great variety of these forms called fungoid growth have been seen and distinguished, and it is demonstrated clearly that certain forms cause certain diseases. For example, cholera or choleraic symptoms have been induced in animals by the introduction of the cholera bacillus into their intestines, and almost invariably the dead animals showed a great abundance of the characteristic bacillus in the intestinal tract. The diseases which are now known or believed to be caused by such virus are very numerous, and directions for preventing a disease from spreading are based upon this theory; epidemic and endemic diseases are generally attributed to such a cause. The list of germ diseases is about as extensive as the list of contagious diseases. EPIDEMIC DISEASES are those that act upon numbers of people at the same time. Probably there is generally, though not always, disease organisms diffused through the air. ENDEMIC DISEASES are confined to particular localities. Sporadic cases of disease are those occurring singly, or scattered considerably. DISINFECTANTS are such substances as act upon the specific germs or minute living particles to destroy them. ANTISEPTICS are such as prevent decomposition or putrefaction. SEPTIC GERMS are generally destroyed when widely diffused in the air. It is believed that oxygen acts as a disinfectant, at least dry air is not favorable to their growth. A dry heat of 300 degrees will destroy them, and they are generally killed by a freezing temperature. Against communicable diseases the chlorine class of antiseptics including iodine, iodoform, bromine, and sulphur are the most effectual, and chlorine and sulphuric acid may without danger be used in the sick room to a sufficient extent to do some good.