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 (see Table 2).
'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 (e.g., rabies); coitus (sexually transmitted viral diseases)). Indirect contact transmission occurs via 'fomites', such as shared eating utensils, improperly sterilized surgical equipment, or improperly sterilized non-disposable syringes and needles.
'Common vehicle transmission' pertains to fecal contamination of food and water supplies (e.g., norovirus 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
Examples of human and animal virus transmission patterns
Mode of transmission
Portal of entry
Influenza virus/influenza Rhinoviruses/common cold Rubella virus/congenital rubella
Hepatitis A virus/hepatitis
Human immunodeficiency virus 1/acquired immunodeficiency syndrome (AIDS) Rabies virus/rabies Russian spring summer encephalitis virus/encephalitis Dengue viruses/dengue Sin Nombre and related viruses
Ebola and Marburg viruses
Leukemia viruses/leukemias (proven only in animals)
Contact/direct/indirect via droplets and droplet nuclei Contact/direct/indirect via droplets and droplet nuclei and fomites Contact/direct/indirect via droplets and droplet nuclei Vertical/congenital Contact/direct/indirect via fomites Contact/direct/indirect via fomites Common vehicle/fecal contamination of water
Common vehicle/fecal contamination of food
Common vehicle/bovine spongiform encephalopathy prion contamination of beef or beef products Contact/direct (sexual)
Contact/direct (sexual), contact/direct
(blood) Vertical/congenital Zoonotic/contact/direct (saliva) Zoonotic/arthropod-borne
Zoonotic/arthropod-borne Zoonotic/contact/direct (rodent urine, saliva and feces) Zoonotic/contact/direct (rodent urine, saliva and feces) Zoonotic/reservoir host unknown; secondary cases contact/direct/ nosocomial and iatrogenic Vertical/germ-line
Transplacental Intestinal tract (oral) Intestinal tract (oral) Intestinal tract (oral)
Intestinal tract (oral)
Intestinal tract (oral)
Genital tract, bloodstream, transplacental, at birth and via breast feeding
Skin (bite wound) Skin (tick bite)
Skin (mosquito bite) Respiratory tract
Respiratory tract and intestinal tract (oral)
Index cases unknown, likely respiratory tract or skin and mucous membranes; secondary cases, contact and iatrogenic (injection) Transmitted as genetic trait indirect contact. Airborne transmission occurs via large droplets and via very small droplet nuclei (aerosols) emitted from infected persons during coughing or sneezing (e.g., influenza) or from environmental sources. Large droplets (>10 pm in diameter) settle quickly, but droplet nuclei evaporate forming dry particles (<5 pm in diameter) which remain suspended in the air for extended periods. Droplets may travel only a meter or so while droplet nuclei may travel over much longer distances.
'Vector-borne transmission' involves the bites of arthropod vectors (e.g., mosquitoes, ticks, and sandflies).
'Iatrogenic transmission' involves health care procedures, materials, and workers (e.g., physicians, nurses, dentists, and veterinarians).
'Nosocomial transmission' pertains to infections acquired while a patient, human or animal, 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 germline 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 in humans, and pestiviruses, such as bovine viral diarrhea virus, in animals.
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