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Epidemiology and Diffusion


Disciplines > Communication > Diffusion > Epidemiology and Diffusion

The chain of infection | Speed and aggression | Mutation | See also


Epidemiology is the study of the spread of disease and uses the same root as the word 'epidemic'. It is interesting that the principles of epidemiology are also applicable in the study of the spread of ideas.

The chain of infection

Etiologic agent

Etiology (or aetiology) is the study of causation, particularly in the spread of disease. The etiologic agent is the germ, the thing that is being spread and which causes the symptoms of the disease.

The virulence of the agent is the severity of infection, which is described by a combination of its morbidity (the chance of getting the disease) and mortality (the chance of dying).

The invasiveness of the agent is its ability to invade tissue. Externally, some agents are easily absorbed by the skin whilst others need to be ingested. Internally, some agents do not spread far within the body whilst others invade all parts.

The reservoir is the place where the agent resides and multiplies, such as the liver or gut. No matter how the agent enters the body, it will head to its reservoir and be based there. Some agents have singular reservoirs whilst others are more invasive.

In the spread of ideas, the agent is the idea itself, the meme. Its virulence is how significantly it affects the person, for example causing serious depression or religious conversion. Its reservoir covers the conceptual subjects, from what you eat to how you drive. An invasive idea may reach all parts of your life, such as a highly prescriptive religion.

Method of transmission

The method of transmission is how the agent moves between hosts.

Direct transmission passes from body to body, through skin contact or bodily fluids. Indirect transmission passes through the through an intermediate stage such as on what an infected host touches. Common vehicle transmission travels in a common medium such as water or food. Airborne transmission occurs where agents travel through the air in water droplets or on dust. Vector borne transmission is carried by a vector, such as salmonella on the legs of flies.

Ideas are spread through various media, from person-to-person to newspapers and the internet.


The disease is spread by transmission between hosts. In disease, this can include animals as well as humans. The organism enters the host through such as the skin, mucous membranes, lungs, gastrointestinal tract, or genitourinary tract.

Ideas aim straight for the brain, where some are based on emotions such as greed or fear, whilst others are more logical and rationally sensible.

Speed and aggression

Diseases need to spread, but they also need to survive. A disease that kills all hosts lasts only as long as the hosts and is consequently not that successful as an evolutionary experiment.


A disease that utilizes dormancy does not show symptoms for some time, during which it may still be able to spread between hosts.

An extreme example of such a disease is AIDS. It lies quietly at HIV for many years infecting all that it can. Then it attacks the immune system, eventually killing its host.

Some ideas spend a long time hanging around, not really being noticed until at last their 'time has come'. The computer mouse was invented in the 1960s but did not really take off until the introduction of MS Windows and the Apple Mac in the 1980s.


Aggressive diseases spread quickly and attack their hosts quickly. They have a high transmission probability and may also have a high fatality.

An example of a very aggressive disease is Ebola, which is highly contagious and very destructive, killing most hosts in a very short time. When detected, isolation methods are the best approach, letting it burn itself out.

Aggressive ideas are 'obvious' but may have a limited lifetime, like a joke that very soon everybody knows and is soon forgotten.


Some diseases remain the same and are susceptible to standard medical treatment. Others mutate regularly, which is why the common cold is so resistant to treatment.

The problem with mutation is that the disease may become less effective with the change. If it is going to mutate then the agent needs to retain an effective core whilst evolving to avoid adaptation by antibodies.

Ideas may evolve too as individual people change them before passing them on to others. Evolutions that do not get passed further will likely die, although, like the many strands of religion, they can end up in competition with one another.

See also

Cause-and-effect reasoning


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