They enter human cells and mimic genetic material, or they enter software systems and mimic existing code. One consumer marketing company, albeit one with lots of money, has perfected the tactic. Everyone from cynical marketers in ad agencies to prison guards in B-movies is using the phrase without irony. Every time they use it, they're endorsing Nike products. Sociologists have long noted that individuals with many casual social connections have a larger influence on communities than do individuals with fewer strong connections.
Viruses thrive on weak ties.
Tupperware, for example, gets someone in a social community, such as a suburban neighborhood, to host a party featuring Tupperware. But the social interaction is funded by the sale of Tupperware products. What's social is indistinguishable from what's commercial a powerful business network built on weak social ties. Viruses do not become epidemics until they reach the tipping point. In other words, the virus must expand through the host population until it reaches a certain threshold of visibility and scale.