Bacteria are either =saprophytes=, _i.e._ they can grow on dead organic or even inorganic matter; or =parasites=, _i.e._ they are dependent for their existence on a living plant or animal which they invade. There are two varieties of each of these, obligate and facultative. An _obligate parasite_ can develop only within a living host; while a _facultative parasite_ can, according to circumstances, lead either a parasitic or saprophytic form of existence. The fact that certain contagia are completely, and others only partially, parasitic brings out important differences in their life-history. Thus, so far as we know, the contagia of scarlet fever, measles, small-pox and hydrophobia do not multiply outside the body. Hence there is a reasonable prospect of annihilating them by measures of disinfection and isolation. The position of diphtheria is doubtful. It may have a saprophytic phase of life. The contagium of tuberculosis, as well as of erysipelas, may have a life outside the host, though to what extent is doubtful. Cholera and enteric fever, although generally communicated by infection, appear sometimes to be communicated by contagia grown in saprophytic life, remote from preceding cases. The infection caused by bacteria may be _local_ or _general_. Thus in tetanus and in diphtheria the invading bacteria usually remain at their original point of invasion (under the skin in tetanus, in the throat in diphtheria). In anthrax always, and often in enteric fever, they are present in the general circulation. In both instances the symptoms of disease are due chiefly to the toxic products or =toxins= formed by the bacteria. These toxins are enzymes, ptomaines, tox-albumins, etc. The specific toxins of anthrax, diphtheria, and tetanus have been identified; and by this means the possibility of neutralising them is created. The =channels of infection=, _i.e._ of invasion of contagia, are the skin and the mucous membranes, particularly of the digestive and respiratory tracts. The =Incubation Period= of an infectious disease is the interval elapsing between the receipt of infection and the earliest development of symptoms. The _period of incubation_ of the chief infectious diseases is shown in the following table:—