A. _From within_ they may enter the tissues either through the inspired air, through food and drink, _i. e._, ingesta, or by means of more direct inoculation, _e. g._, by foreign bodies or by venereal contact. The danger through infection by inspired air is very small, and concerns probably a limited number of organisms, of which the tubercle bacillus is the most important. Foul air and air which emanates from sewers, cesspools, etc., while most unpleasant to breathe and deleterious in many other ways, do not necessarily contain any microörganisms which can be injurious. This fact, in opposition to general belief, is, nevertheless, proved by recent investigations. The ingesta furnish the most fertile source of contagion from within, but the diseases thereby produced fall for the most part into the domain of medicine rather than that of surgery. B. _Infection from without_ the body may come by actual contact with previous skin or mucous lesions, and particularly from noxious insects and certain parasites. Among surgeons the principal sources of contact infection to be enumerated and guarded against are: 1. Skin and hair; 2. Instruments; 3. Sponges or their substitutes; 4. Suture materials; 5. The hands of the surgeon and his assistants; 6. Drainage materials; 7. Dressing materials; 8. From miscellaneous sources, _e. g._, drops of perspiration, unclean irrigator nozzle, a contaminated nail-brush, the clothing of the operator, etc. While insisting here upon the recognition of these sources of danger, the precautions to be taken against them are to be considered under another heading, to which the reader is referred. One of the greatest sources of possible infection has of late been shown to be the presence of flies and other noxious insects, which act as carriers of infection. The Egyptian ophthalmia, which ruins the sight of 30 per cent. of the inhabitants of Egypt, has been shown by Howe and others to be due to infection by this mechanism; and a simple bacteriological experiment will suffice to show that the foot-tracks of a single fly across a wound furnish abundant opportunities for infection with organisms which are presumably virulent. In fact, the danger of carriage of infection by this means is greater than from almost all other sources, except the use of improper materials during surgical operations. CLASSIFICATION OF INFECTIONS. We speak of infections as _primary_, _secondary_, and _mixed_; and it is necessary, for purposes of accuracy at least, to make a reasonably clear distinction between them. =Primary Infection.=--By primary infection is meant infection with a single form of organism whose effects are prompt and speedy. Of this, erysipelas or syphilis may serve as illustrations. Most of the acute infections belong to the primary type.