By infection is meant the successful invasion of the tissues by an organism. The mere presence of the living agent within the body is not sufficient to cause infection; it must enter the tissues and give rise to symptoms that indicate a diseased condition. Trypanosomes may be even within the blood-vessels of certain animals and cause no symptoms.

There are normally many organisms contained within the body, particularly in the alimentary canal, but they give rise to no pathologic conditions until they leave their accustomed habitat.

Infection, therefore, means the entrance of organisms into the body, with subsequent injury to the tissues involved. By an infective disease is meant one that is the result of the entrance into and the multiplication of the organisms within the body.

The symptoms in such a condition are the results of the formation of toxins, and not of mechanical disturbances. As a rule, no symptoms appear immediately after the entrance of the invading organism into the body, as there is not sufficient toxin present. The interval between the inoculation and the symptoms resulting from the toxins is known as the period of incubation, which differs greatly in different diseases.

Then, too, infection may be influenced by certain peculiarities of the infecting organism and of the attacked individual. It is a well-recognized fact that true infection does not always occur after the primary invasion. This may be due to variations in the ability of the micro-organism to produce disease. Some have very little power to multiply after gaining entrance into the tissues, but they may form large amounts of poison. Other bacteria may form but little poison, yet have almost unlimited powers of multiplication when in the body. The number of the organisms and the mode of entrance also effect the severity of the infection.

The infecting organisms may come from outside of the body - exogenous. They may enter the lungs in consequence of impure air or they may gain entrance into the body along with the food or water. Wounds of various sorts may carry the organisms into tissues; then, too, it has been discovered that diseases may be conveyed from one person to another by biting insects.

Endogenous infections are those resulting from organisms that are commonly present within the body. They may be due to some change in the tissues of the host, that allow these living bodies to escape from their normal surroundings and gain entrance into unusual localities. The colon bacillus that is normal in the intestinal canal may cause much trouble if it gets into other localities. It must be remembered that practically all openings communicating directly or indirectly with the external air will contain bacteria. They are normally present in the skin and the adjacent mucous membranes. The mouth and the intestines contain many varieties, while the stomach, on account of its acid contents, contains but few. The normal lungs are free from bacteria.

The results of bacterial invasion are much influenced by the local conditions at the point of attack - the avenue of infection. These are of the greatest importance in determining the occurrence or non-occurrence of infection. Certain bacteria, such as those of cholera, typhoid, and dysentery, attack only the intestinal canal, and will not cause trouble unless they first gain access to that tract. The gonococcus will not produce an infection in skin, even if that tissue be wounded. The tubercle bacillus in the lung can cause widespread destruction; in the skin little appears other than a localized tubercle.

Of the various obstacles to infection, an intact epithelial covering of the body inside and out is probably the most important. Such a covering is an efficient barrier against staphylococci and streptococci. An injury, nevertheless, need be but very slight in order to permit the organisms to enter. Such an injury may be secondary to the destructive action of products formed by the bacteria themselves. Associated infections may, likewise, be of importance. Tetanus organisms may not survive if inoculated alone into normal tissues, but will grow if pyogenic bacteria are also present. The same probably holds good with the diphtheria bacillus. Both of these, although unable to grow and multiply in an intact structure, can do so when these tissues have been previously or simultaneously bruised or lacerated. Although the epithelial coverings can protect, yet injuries to them are very common, and bacteria frequently gain entrance to the tissues. The question then arises, Why does not every infection become generalized and lead to the destruction of the host? There are two factors concerned in this problem. One is the aggressive or attacking force of the micro-organism, the other is the resistance which the host offers to the presence of the invader, to its multiplication, or to its ability to produce harmful substances. This resisting power is the defensive force.