Virus transmission may be horizontal or vertical. The vast majority of transmission is horizontal, that is between individuals within the population at risk. Modes of horizontal transmission of viruses can be characterized as direct contact, indirect contact, common vehicle, airborne, vector-borne, iatrogenic and nosocomial. Vertical or transplacental transmission occurs between the mother and her fetus or newborn. Some viruses are transmitted in nature via several modes, others exclusively via one mode (Table 1).
Direct contact transmission involves actual physical contact between an infected subject and a susceptible subject [e.g. kissing (Epstein—Barr virus, the cause of mononucleosis), biting (rabies), coitus (sexually transmitted viral diseases)]. Indirect contact transmission occurs via fomites, such as shared eating utensils, improperly sterilized surgical equipment or improperly sterilized nondisposable syringes and needles. Common vehicle transmission pertains to fecal contamination of food and water supplies (e.g. Norwalk virus diarrhea). Common vehicle transmission commonly results in epidemic disease. Airborne transmission typically results in respiratory infections (and less typically in intestinal infections), but these infections may also be transmitted by direct and indirect contact. Airborne transmission occurs via droplet nuclei (aerosols) emitted from infected persons during coughing or sneezing (e.g. influenza) or from environmental sources. Large droplets settle quickly, but microdroplets evaporate forming dry droplet nuclei (less than 5 nm in diameter) which remain suspended in the air for extended periods. Droplets may travel only a meter or so whereas droplet nuclei may travel much longer distances. Vector-borne transmission involves the bites of arthropod vectors (e.g. mosquitoes, ticks, sandflies). Iatrogenic transmission involves health care procedures, materials and workers (e.g. physicians, nurses, dentists). Nosocomial transmission pertains to infections acquired while a patient is in hospital.
Vertical or transplacental transmission occurs from mother to fetus prior to or during parturition. Certain retroviruses are vertically transmitted in animals via the integration of viral DNA directly into the DNA of the germ-line of the fertilized egg. Other viruses are transmitted to the fetus across the placenta, yet others are transmitted when the fetus passes through the birth canal. Another vertical transmission route is via colostrum and milk. Vertical transmission of a virus may or may not be associated with congenital disease (i.e. disease that is present at birth) which may be lethal (and the cause of abortion or stillbirth) or the cause of congenital abnormalities. The herpesviruses, especially cytomegaloviruses, and rubella virus cause important congenital diseases.
Enteric infections are most often transmitted by direct contact and by fomites in a fecal-oral cycle that may include fecal contamination of food and water supplies; diarrheic feces may also splash to give rise to aerosols. Respiratory infections are most often transmitted by the airborne route or by indirect contact via fomites in a respiratory cycle, i.e. virus is shed in respiratory secretions and enters its next host through the nares during inhalation. The respiratory cycle is responsible for the most explosive patterns of epidemic disease.
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