The chief danger from wounds is that their surfaces may become infected.  Death from wounds is due more frequently to infection than to the actual injury represented by the wounds.  Much depends upon the character of the wound.  Infection of clean wounds which are made by a sharp cutting instrument and from which there is abundant haemorrhage with sealing of the edges of the wound by clotted blood, rarely happens.  Typical wounds of this sort are often made in shaving, and infection of such wounds is extraordinarily rare.  If, with the wound, pathogenic organisms are placed in the tissue, or foreign substances such as bits of clothing are carried in with a bullet, for example, or if the instrument causing the wound be of such a character as to produce extensive lacerations of tissue, infection is more apt to occur.  The less frequency of infection in modern wars is in part due to the simpler character of the wounds and in part to the fact that modern fixed ammunition is practically free from germs.  The old spear-head, the arrow, the cross bow bolt, had little regard for the probabilities of infection.  Whether infection follows a wound depends both upon the entry of pathogenic organisms and upon these finding in the tissues suitable opportunities for growth.  In wounds in which there is much laceration of tissue organisms find the most favorable conditions for development.  The very slight wounds produced by the exploded cap in the toy pistol give suitable conditions for the development of the bacillus which produces tetanus or lockjaw.  The deaths of children from lockjaw following a Fourth of July celebration have often exceeded the total deaths in a Central American revolution.  The tetanus bacillus is a widely distributed organism, whose normal habitat is in the soil and which is usually present on the dirty hands of little boys.  The toy-pistol wounds are made by small bits of paper or metal being driven into the skin by the explosion of the cap.  The wound is of little moment, the surface becomes closed, and a bit of foreign substance, a few dead cells and the tetanus bacilli from the surface remain enclosed and in a few days the fatal disease develops.  Infection of the surfaces of old wounds such as the surface of an ulcer takes place with difficulty.  Large numbers of leucocytes which give protection by phagocytosis are constantly passing to the surface, and there is also a constant stream of fluid towards the surface.  On such a surface there may be an abundant growth of pathogenic organisms, but no infection results.

In most infections there is a focus where the infectious organisms are localized; this may correspond to the point of entrance on a surface or it may be in the interior of the body, the organisms being deposited there after entrance.  At this primary localization, the atrium of infection,[1] the organisms multiply and from this point further invasion takes place.  Many secondary foci may be formed in the organs by