Local conditions are of the greatest importance in determining the occurrence or non-occurrence of infection. Cholera, typhoid, and dysentery attack the digestive tract alone. The gonococcus apparently can invade only through the mucous membranes of the genito-urinary apparatus or of the eye. The tubercle bacillus, although able to invade any tissue of the body, gives rise to modified forms of the disease. If it invades the skin a local condition known as lupus occurs. This may last for years without becoming general. The same organism entering the lungs can give rise to consumption, with extreme destruction of tissue and generalized infections. Skin infections in general tend to remain localized unless the organisms have been implanted quite deeply, so as to more readily gain access to the circulation.

The chief obstacle to infection no doubt lies in the integrity of the epithelial coverings of the body, both inside and out. An injury, however, need be but very slight in order to allow the micro-organism to gain entrance.