ROUTES AND MODES OF INFECTION
Respiratory Route
Pathogenic
organisms may be inhaled. This mechanism by which
infection is spread is especially significant in many respiratory diseases and is responsible for large epidemic outbreaks. Viral influenza is spread by tiny, airborne particles that can reach the lower airways of the lungs. Tuberculosis, smallpox, and measles are spread by contamination of the air by infected patients. In contrast, fungal diseases such as histoplasmosis and coccidioidomycosis are acquired by inhaling infectious particles derived from soil harboring the
organisms, and Legionnaires disease is spread by contaminated water droplets.
Gastrointestinal Route
Infections may be acquired by ingestion of the causative organism. Infectious hepatitis, poliomyelitis, and typhoid fever are transmitted by this means. The infecting dose varies with the disease. In some conditions as few as one or two organisms can initiate infection, whereas in others as many as a million organisms may be required. Certain features of the person encountering the organisms may potentiate their effects. For example, Salmonella typhi is the causative organism of typhoid fever, and persons who have a lack of acid production in their stomachs can be infected by far fewer of these organisms than persons with normal acid production.
Direct Contact
Mucous membranes may be a portal of entry for many infections. The venereal diseases gonorrhea and syphilis are transmitted by direct contact of mucous membrane to mucous membrane. This may involve penis, vagina, urethra, mouth, or anus. Although the skin is an impenetrable barrier for these and most infections, certain organisms may break through. For example, staphylococci may cause boils on skin that is normal or has only a trivial irritation. The streptococcus may cause cellulitis (a spreading inflammation) or impetigo (a crusting lesion) on skin that is intact.
Mother to Child
It is possible for a fetus to become infected while in the uterus. Therefore, infections that the mother acquires during pregnancy may damage the fetus. Some of the most dangerous of these include rubella (German measles), cytomegalovirus disease, and toxoplasmosis. All these conditions produce mild disease in the pregnant woman but may result in devastating damage to the developing fetus. In addition, infants may be infected at birth during passage through an infected birth canal. This can result in congenital (present since the time of birth) gonorrhea, syphilis, herpes viral infections, or streptococcal infections.