The Contagion of Ideas


June 2011
Ivan Obolensky

By equating the spread of ideas with that of the contagion of disease and examining their similarities and differences, one can gain a better understanding of how ideas permeate through society, and how one can get one’s own ideas across to others more effectively and to more people.

Certainly ideas can be infectious. A fad, for example, seems to sweep through a society just like an infectious disease. Consumers get the idea that they have to have the new X gadget from a well-known brand. When the release date approaches, lines stretch outside the store awaiting the new gadget. New owners show their friends their new purchase, and their friends have to have one just like it. Before long everybody has one. The point here is not that the new gadget is harmful, but that the idea of “want” and “need” spreads from person to person in a way similar to an airborne virus. In some cases, people cannot stop thinking about the new gadget and feel compelled to go out and get it no matter what. It might even be said to involuntarily infect their minds.

One of the consequences of the computer age has been the collection and storage of vast quantities of data. These data can be collected, grouped, and massaged to show patterns and tendencies. As the number of data increase, computer programs can be created that can better model how a population whether human, biological, or physical, will act in the future. The stopping characteristics of a particular car and the spread of a disease through a city can both be modeled. Even though the models can appear oversimplified compared with the myriad of real-world inputs that seem to affect a situation, by knowing just a few key parameters one can discover how many overall behaviors can change over time.

Not all things are easy to model. Some organisms and their interrelationships involve complex feedback and feed-forward mechanisms that make outcomes almost impossible to predict with accuracy particularly in the long run. Predator-prey relationships and interest rate behavior are like that. Others, such as the number of deaths in a large population, are easy to model in the long run but hard to model on the short term.

In the case of the spread of ideas and the spread of an infectious disease, both appear to follow a similar pattern.

The standard model for the spread of an infectious disease is the SIR model (where SIR stands for: Susceptible, Infectious and Recovered). The SIR model is based upon the idea that a population is divided into those who are able to be infected, those who have the disease and are spreading it, and those who are recovered and are immune. The number of people in each category (unexposed, infected, and immune) changes as the disease passes through a given population. Each specific disease does this in different ways: among them, how it is transmitted and how fast? What its life span is and how long it will remain infectious? These characteristics taken together describe how virulent a disease is.

In the SIR model, this overall characteristic is called the disease’s Basic Reproduction Number (R). This number measures how many other individuals a disease carrier will infect before he, or she, is no longer contagious. The number can run from zero on up and is usually different for each disease. As an example, the Reproduction Number of AIDS is 2 to 5, measles: 12 to 18, with malaria over 100 provided a mosquito population exists to transmit it. The fact that malaria has such a high R means it is extremely hard to combat because it can spread so easily, even if by non-human to human means. The R of 100 indicates that for every person infected, one hundred more will be infected. Is it any wonder that malaria is one of the planet’s worst diseases? 1

Like an infectious disease, ideas, too, are transmitted from one person to another. Of course, not all ideas are similar. There can be those one just heard of and those one read in a book. There can be brilliant ideas, or stupid ones. Here, this article is about immediate ideas as they play out, such as a rumor, or a really bright idea. Even ideas like the iPad; ideas that seem to spread through society easily, initially required constant reiteration and nurturing by the inventor and by those who wanted it to catch on in order to become self-sustaining and wildly successful.

It is no wonder advertising talks about campaigns. Messages must be repeated many times over an extended period in order to remain in the public mind, or they would simply fade into oblivion. One interesting point with the iPad is that it is somewhat similar to a retro virus in that the iPad transforms into a channel for other ideas and becomes part of the infrastructure of idea transmission, thus ensuring its survival. Regardless, to be successful, an idea has to spread.

If your idea, or creation, has a Reproduction Number greater than one, then the concept is spreading through society. A person receiving your idea passes it on to more than one person before they forget about it, or turn to other things. A high enough R and it acts like an epidemic. Facebook as an idea and ultimately as a channel for spreading more ideas has an R greater than one and probably closer to 5-7 if not higher, since on average, a person has at least five to seven close contacts. Facebook subscribers will continue to increase as a result, until its use saturates the available population at which point it will decrease as subscribers become less enchanted with social media in general. Just like a disease as it burns through a population leaving behind those who are immune. Of course, Facebook can continue to reinvent itself.

If it can become the channel that spreads ideas like a retro virus, it has the potential of being self-sustaining, like the idea of a telephone, but even then it will probably have to reinvent itself over and over. After all, landlines turned into cell towers. Being the hottest thing as opposed to a conduit requires a tremendous amount of almost continuous reinvention in order for the business to remain viable and for the idea to remain in the public consciousness—both contributing factors to maintaining a high Reproduction Number.

If the R is less than one, the disease or idea is not sustainable because each diseased person is not infecting at least one other. Fewer and fewer people are being infected until no one is. Floppy disks, pet rocks, and Assyrian as a spoken language, all had an R less than one at some point.

If the Reproduction Number is exactly one, then every diseased person will infect only one other person before becoming non-infectious creating a constant number of those who have it. The disease will be said to be endemic in the population. It can exist and will continue to exist in the population, although at a steady state.

The idea of restaurants putting salt and pepper shakers on the table seems to be constant. In other words its R is equal to one. As far back as the nineteenth century, it was customary to set the formal table with “salt cellars” (tiny bowls of salt accompanied by a tiny spoon used to sprinkle the salt over food). This idea saturated its environment and became constant, passed down to each generation. However, with warnings of increased heart attack risk, and the rise of the low-sodium-diet idea, perhaps the salt-shaker-on-the-table days are numbered, and its R is now less than one.

We can surmise then that an R, for an idea at least, can change as its environment changes and as the idea comes into competition with other ideas. In the instance of a fad, the R is high, and the idea can flash through a population rapidly. To an individual infected with a particularly persistent idea, getting rid of it may be similar to experiencing withdrawal symptoms or a having a compulsion. It is likely to last five to seven days until its force diminishes. This time span is similar to the effects of drug withdrawal according to several addiction advice sites, and the elimination half-life of many drugs. The elimination half-life is the time it takes a substance to lose half of its pharmacologic or physiologic activity).2, 3

Newsworthy items or ideas that are still attention-grabbing seem to remain in the public consciousness for an average of about three days, judging from how long a typical item is repeated in the news. It is, therefore, likely that an idea will have a life span of a week maximum, before it loses focus and is replaced by other more current, or now more relevant, ideas and thoughts in the mind of the individual. This is a key concept because in the SIR model, there are several factors that seem to determine the Reproduction Number and thereby how infectious a disease is. One of them is the length of the infectious period.

Most infectious diseases have a time when the victim is contagious. This usually lasts for at least a couple of weeks. SARS (Severe Acute Respiratory Syndrome) had an infectious period estimated at only a week and may be one reason for its speedy disappearance (besides efforts by health services to combat it.)4

This is also one of the areas where the efficiency of the spread of a disease and that of an idea differ. An idea is most likely contagious for only three days. Compulsive and attention-sticky ideas may last seven days. Successful diseases have much longer periods when the carrier is infectious.

Ideas, of course, can last longer than a week. They can be written down. They can be reintroduced. One can be reminded, but here we are looking at new ideas that are on the forefront of the mind. We are looking at hot ideas that are spreading from one person to another quickly. More mundane ideas like “I have to take out the trash on Thursday” can last in the mind a few seconds and be completely forgotten unless written down. It is also unlikely to be spread to someone else. It is just not infectious.

The infectious period for a disease lasts at least a couple of weeks; ideas, as noted, have a much shorter time when they are active in the mind and are likely to be passed on. Ideas in and of themselves are not self-sustaining because they circulate in the mind for such a limited time. They do not have sufficient longevity in their hosts to create an “idea” epidemic. With a few exceptions (see below) rarely does one pass on a new idea to more than five people which would be the minimum needed for an idea to duplicate the average Reproduction Number of diseases in general (which is around five).5

It is interesting to note the rise of tweeting and re-tweeting. Ideas spread in this fashion and on social media sites can have a significantly higher R and act like a highly contagious disease in terms of its spread. Only the population of those following a given tweeter is small in comparison to the population as a whole, and tweeting as an activity is not as widespread as it perhaps will be, limiting its spread. Further, only 22.5% of users accounted for about 90% of all activity.6 There is no doubt that the rise of the Internet and social media has significantly boosted the possibility of an idea spreading and acting like a worldwide epidemic, as the number of possible interactions and number of possible connections increases in the future.

One can also gather lessons from the spread of disease and apply them to idea-spreading, itself. For a disease to spread more effectively, it has to increase its Reproduction Number. In other words it has to increase its propagation.

Within the framework of ideas, if one correlates the Reproduction Number to the quantity and quality of the message one is trying to get into the public domain, one can see that the more an idea is communicated, the more it will remain alive in the target population. One also has to be aware of the tremendous amount of effort that will be required, even for a good idea to move through society. Almost every idea, except the most electrifying, will not be self-sustaining because ideas in general do not stick around long enough in the mind to really give it a base. It is no wonder that Madonna and Lady Gaga have been successful. Their behavior is designed to foster a greater longevity in the mind of the public. How much chance does the idea of buying your product/service have, when placed against the idea of either of these women? One can create excitement about a product, but there are limits.

One can take a page from the book of disease transmission by increasing population size to overcome the “not-quite-hot-enough idea” with no possibility of a re-tweet. The chicken pox virus starts with a limited population: children. At a later time, the same virus can infect the adult population as shingles. This can work with ideas as well. Creating a new population for one’s idea or concept might be much more effective at idea transmission than trying to make it have more impact. The time span of idea retention is a constraint that cannot be changed in most cases.

A simple and effective way to do this is to consider translating one’s message into another language.

Since one is not able to change how long the mind on average retains ideas, and unless one has an idea that borders on the compulsive such as the ranking of college girls among college boys as in Facebook, it would appear that utilizing the multi-language attributes of the world we live in is the simplest and most cost-effective method of making ones ideas better known in the world.

1 Keeling, M. (2001, March 1). The Mathematics of Diseases: Issue 14. Retrieved June 8, 2011, from Plus Magazine …living mathematics:

2 (2010, December 1). Question: How long does the withdrawal symptoms last? Is it a good idea to quit smoking while trying to kick opiates? Retrieved June 8, 2011, from

3 Segen’s Medical Dictionary. (n.d.). Biological Half-Life. Retrieved June 8, 2011, from The Free Dictionary:

4 Lipsitch, M., Cohen, T., Cooper, B., Robins, J. M., Ma, S., James, L., … Murray, M. (2003, May 23). Transmission Dynamics and Control of Severe Acute Respiratory Syndrome. Retrieved June 8, 2011, from Science: The World’s Leading Journal of Original Scientific Research, Global News, and Commentary:

5 Strain, S. (n.d.). SIR Epidemic Dynamics. Retrieved June 8, 2011, from Wolfram Demonstrations Project:

6 Cheng, A. & Evans, M. (2009, June). Inside Twitter: An In-Depth Look Inside the Twitter World. Retrieved June 8, 2011, from Sysomos Resource Library:

If you would like to sign up for our monthly articles, please click here.

Interested in reprinting our articles? Please see our reprint requirements.

© 2011 Ivan Obolensky. All rights reserved. No part of this publication can be reproduced without the written permission from the author.

Leave a Reply