Ideas, products, messages and behaviours spread just like a virus: they are passed from one person to another, rapidly infecting more and more people. An example of this uncontrolled spread is yawning. Just reading that word makes you yawn. Anyone who sees or hears you yawn, will also yawn and so on until it has run its course.
Most “epidemics” never eventuate because the innovators and early adopters who embrace new ideas, products, messages and behaviours are fundamentally different from everyone else. Therefore, the message must be something that can be passed on and “sticky” enough that your target market find it memorable.
Small changes can make the “virus” more infectious very quickly. Therefore, you need to abandon the idea of proportionality. A successful epidemic only requires three things
An infectious agent,
Those who are extraordinarily active transmitting the agent, and
An environment that supports the agent’s spread.