Among the first to write about viral marketing on the Internet was the media critic Doug Rushkoff.
The assumption is that if such an advertisement reaches a "susceptible" user, that user becomes "infected" (i.e., accepts the idea) and shares the idea with others "infecting them", in the viral analogy's terms. As long as each infected user shares the idea with more than one susceptible user on average (i.e., the basic reproductive rate is greater than one—the standard in epidemiology for qualifying something as an epidemic), the number of infected users grows according to an exponential curve. Of course, the marketing campaign may be successful even if the message spreads more slowly, if this user-to-user sharing is sustained by other forms of marketing communications, such as public relations or advertising.