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Routes for Spread of Infectious Disease   To Infectious Disease theme page


1. Core Knowledge:

Think, first, of the "epidemiologic triad" of a disease agent, a susceptible host, plus an environment that brings the host and agent together.

Disease requires all three to arise, and to spread.

(Note, this can apply to infectious and non-infectious diseases)

Epidemiologic triad of agent, host and environmental factors.

There are six common ways in which infectious agents spread:

Direct Direct physical contact (body surface to body surface) between infected or colonized individual and susceptible host.
   Examples of transmission: Shaking hands; kissing; coitus.
   Examples of infections: Common cold; sexually transmitted diseases.
   Precautions: Hand hygiene; masks; condoms.
Indirect Infectious agent deposited onto an object or surface (fomite) and survives long enough to transfer to another person who subsequently touches the object.
   Examples of transmission: Not washing your hands between patients; contaminated instruments
   Examples of infections: RSV; Norwalk; rhinovirus; perhaps influenza.
   Precautions: Sterilizing instruments; disinfect surfaces and toys in school.
Droplet Contact, but transmission is through the air. Droplets are relatively large (>5 µm) and projected up to about one metre.
   Examples of transmission: Sneezing; coughing, or (in health care) during suctioning
   Examples of infections: Meningococcus; pertussis; influenza (maybe: there is some debate); respiratory viruses.
   Precautions: Masks; cover mouth; move school desks more than a metre apart; side-step that cougher! Special ventilation not required as aerosolization does not occur.
   Note for Nerds: sometimes you will see droplet transmission classified in a separate category intermediate between contact and non-contact. This makes good sense, but do not lose sleep over such debates.
Non-contact Airborne Transmission via aerosols (airborne particles <5µm) that contain organisms in droplet nuclei or in dusts. Can be spread via ventilation systems.
   Examples of transmission: Via ventilation system in a hospital;
   Examples of infections: TB; varicella; measles; chickenpox; smallpox (and maybe influenza: controversial, as more likely via droplets).
   Precautions: Masks; negative pressure rooms in hospitals
Vehicle A single contaminated source spreads the infection (or poison) to multiple hosts. This can be a common source or a point source.
   Examples of transmission - Point source: Food-borne outbreak from infected batch of food; cases typically cluster around the site (such as a restaurant); IV fluid; medical equipment.
Common source: The cyanide poisoning of Tylenol in Chicago in 1982. Cases may be widely dispersed due to transport & distribution of the vehicle (here, a medication)
   Precautions: Normal safety and disinfection standards. Package medicines in tamper-proof containers.
Vector borne Transmission by insect or animal vectors.
   Example of infections: Mosquitoes and malaria.
   Precautions: Protect via barriers (window screens, bed nets); insect sprays; culling animals.

Link: Public Health Agency Guidelines on containing hospital infections (see the downloadable pdf document)
           Guidelines on pandemic planning

2. Nice to Know:

Terms describing spread of an infection

To survive and spread, an infectious agent has first to enter the host, then reproduce itself, and then find a way to spread to other hosts. These various stages have different names, but unfortunately the terms are used somewhat differently by different authorities. Here is a diagram that tries to clarify the stages in the progression of an infectious disease:

Terms describing stages in an infectious disease

3. Additional Information:

Linking the Mode of Transmission to Virulence

Virulence is the harmfulness of an infection, the degree to which it destroys its host.  Virulence varies with the mode of transmission:

Ewald has argued that we might exploit this phenomenon and actively encourage organisms to evolve less virulent forms, rather than escalating the fight by producing increasingly strong antibiotics.  Protecting the water supply is good, as fewer people get sick from dirty water; but an additional benefit might be that the prevalent strain of cholera will become less virulent because transmission will depend on people remaining mobile for it to spread.  [PW Ewald. Evolution of infectious disease. New York: Oxford University Press, 1994.]

So, this helps explain why infections cause irritating symptoms:

Paul Ewald has a video talk on pathogenicity

Updated October 14, 2014