(1) Some can only be propagated by _inoculation_—the introduction through an abraded surface of a minute quantity of the poison; as in glanders and hydrophobia. Others, again, _may_ be introduced in this way, but are usually acquired in another manner, as scarlet fever, small-pox. (2) Some are carried through the _atmosphere_. The contagium of small-pox can be carried as far as any, while that of typhus fever only traverses a few feet. The atmosphere acts as a conveyer of infection, and the infectious matter must necessarily, in most instances, be in the condition of dust to enable it to be wafted by currents of air or disturbed by the movements of persons in an infected room. (3) _Clothes_, books, and furniture are not uncommonly carriers of infection. An old letter, or a lock of hair, has even after many years’ concealment in an enclosed space produced infection on being brought to light. Woollen articles convey infection more easily than calico, and dark clothes better than light coloured. A fever nurse’s clothes should never be woollen, but some washable material. (4) _Drinking water and food_ often form a vehicle for infection. Milk and water are the two usual sources of infection; but uncooked food, especially oysters and mussels, fed in sewage-polluted estuaries, may produce the same effect. Cholera, enteric fever, dysentery, and summer diarrhœa are the chief diseases from this source; but scarlet fever and diphtheria occasionally have a similar origin. Milk may be infected from having been handed by an infectious patient; or it may possibly convey infection of the disease from which the cow at the time is suffering, _e.g._, tuberculosis (see also page 312). Water may be contaminated with sewage or the excreta of a single infectious patient. 2. _They retain their specific character and origin._ Small-pox never produces scarlet fever, nor _vice versâ_; and it is found universally that all the specific fevers “breed true,” each one retaining its identity. More than this, a previous case of the same fever can nearly always be detected on careful examination. Overcrowding and other insanitary conditions diminish the resistance to infection, and may increase its virulence. 3. _The behaviour of contagia_, when received into the system, is characteristic of these diseases. There is first of all a period of latency or _incubation_, during which no symptoms are manifested (see page 287.) The incubation period is followed by the characteristic symptoms of the particular fever, which disappear in a variable period, leaving the patient, as a rule, more or less _insusceptible to a second attack_ (see page 288).