Modes of Transmission

  • How does an infection spread?

    Epidemiology is the study of disease transmission. The spread of disease involves the following components:

    1. A susceptible host
    2. A biological agent sufficient to cause disease
    3. A mode of transmission

    All three components must be present for an infection to spread from one person to another. This is what is called the 'chain of infection.'

    An organism may have a single route of transmission, or it may be transmissible by two or more routes. If any one is missing, the chain is broken and the potential for infection is eliminated.

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    Why should I know about disease prevention?

    Knowing the methods by which a disease is transmitted is important in infection control and in attempting to decrease nosocomial infections. It also demonstrates the importance for hand hygiene in the 'chain of infection.'

    The most common modes of transmission that occur in the hospital environment are contact, droplet and airborne.

    1 - Contact spread

    In contact spread, the susceptible person has contact with the infected source or reservoir and the contact is either direct, indirect or by droplets.

    1. Direct patient contact
      Where there is actually physical contact between the source and the susceptible person. The source and the victim need to be in close contact to result in exposure to skin and body secretions. The organisms can be transmitted from one part of a person's body, such as their skin, to another part of their own body or to another person. e.g. touching a wound
       
    2. Indirect patient contact
      This occurs when organisms from an infected host or other reservoir are transmitted to a susceptible host via an inanimate object. In a hospital environment, fomites can be contaminated and act as a source of infection. These include medical equipment, clothing, bedding, dressings and sinks. Gastrointestinal pathogens such as Salmonella can be transmitted in this way.
       
    3. Droplet transmission
      The transmission of infectious agents in droplets expelled from respiratory secretions by coughing, sneezing or talking, is another form of contact transmission. Droplets are large particles that rapidly settle out on horizontal surfaces or deposited on susceptible person's conjunctivae, nasal membranes or mouth. They cannot be transmitted beyond a radius of several feet from the source. Pathogens that are transmitted in this way are the cold influenza viruses and Respiratory Syncytial Virus (RSV). 

    2 - Airborne spread

    Airborne describes organisms have a true airborne phase in their route of dissemination. This usually results in distance more than several feet between the source and the victim.

    Pathogens that are transmitted in this way are Varicella (chicken pox) and tuberculosis (TB)

    3 - Host

    The 3rd link in the chain is the host or victim. Whether or not the micororganism infects a person depends on the ability of the organism to cause disease and the ability of the body to resist it. Previous exposure and immune response will also play or role. Disease does not always follow the transmission of infectious agents to the host.

    How do we prevent the spread of infection?

    To prevent the spread of infection it is therefore necessary to eliminate at least one of these elements.