Transmission (epidemiology)

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Transmission, in the context of infectious disease, implies the passing of a pathogen from one host or reservoir of the pathogen to a new susceptible host. The term transmission is also used for the transfer of genes in genetic diseases.

Instances of transmission can be divided into direct or indirect, where a pathogen is transferred through an intermediate object or medium e.g. a contaminated ultrasound probe. Understanding the types of transmission possible for a specific disease is critical to control disease spread. The mode of transmission does not always imply the specific portal of entry in the affected person e.g. in vertical transmission, a disease may pass from mother to child during childbirth, by skin to skin contact or blood.

Types or modes of transmission include:

  • airborne
    • aerosol
    • droplet
  • vertical: from mother to fetus
  • horizontal: across generations, e.g. between siblings
  • sexual
  • orofecal
  • iatrogenic
  • waterborne
  • fomites

NB: When epidemics of emerging diseases occur all the modes of transmission may not be apparent.

History and etymology

The words transmission and transmit are derived from the Latin word transmissio which meant sending across.