PATHWAYS OF INFECTION
The various routes by which organisms gain entrance to the body are protected by defense mechanisms. If these mechanisms are breached in some way, infec-tion can occur. Once inside the body, the infection may spread directly via the bloodstream or lymphat-ics. One method of containing and preventing infec-tions is to protect these pathways and maintain the barriers. Think of the various ways by which this pro-tection can be achieved as the different pathways of infection are addressed.
Keratinized epithelium, sweat, and sebum are some protective barriers present in skin. When the conti-nuity of skin is breached by laceration, burns, or re-duced blood supply, the barrier is no longer effective, and the risk of infection increases.
Infection may be acquired through the skin by di-rect physical contact (e.g., herpes simplex, ringworm,impetigo), by infection when the skin is disrupted (e.g., tetanus), orby injection into the skin by vectors carrying infectious agents (e.g., malarial parasites in-jected by mosquitoes). Infections may also be ac-quired by injection by humans,such as transfusion of infected blood and blood products and contaminated needles. Certain infections are spread by direct pene-tration of the skin by the infectious agent(e.g., hook-worm larvae).
Infection may also be transmitted by indirect con-tact with infected body fluids via towels, shared uten-sils, or bedding.
Entry of pathogens through the respiratory tract is prevented by the presence of mucus and cilia that move the mucus toward the mouth. Defense cells, an-tibody secretions, and lymphoid tissue (tonsils) in the mouth and pharynx also protect the respiratory tract. Depression of the cough reflex by drugs; interference with ciliary transport, as in alcoholism, cold, and loss of ciliated cells as a result of smoking; and bronchial obstruction as a result of various causes can all con-tribute to the weakening of the barrier and an in-creased risk of infection. Inhalation of droplets car-rying infectious agents is the usual mechanism of transmission.
This tract is protected by lysosomes, antibodies pre-sent in various secretions and the pH of secretions that are not conducive to growth and multiplication of mi-croorganisms. The mucosal lining and the growth of natural intestinal flora in the colon also serve as a pro-tective barrier. Entry of infectious agents through the gastrointestinal tract is via infected food and drink, in-cluding fecal contamination.
This tract is normally sterile. But risk of infection is increased by obstruction of urinary flow, catheteriza-tion, and alteration in normal flora by prolonged use of broad-spectrum antibiotics and others. Entry of infection through this tract is more common in women because of the shortness of the urethra.