In viral marketing

Viral marketing

Viral marketing, viral advertising, or marketing buzz are buzzwords referring to marketing techniques that use pre-existing social networks to produce increases in brand awareness or to achieve other marketing objectives through self-replicating viral processes, analogous to the spread of viruses...

 or software application design the K-factor can be used to describe the growth rate of viral apps. This usage is borrowed from the medical field of epidemiology

Epidemiology

Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...

in which a virus having a k-factor of 1 is in a "steady" state of neither growth or decline, while a k-factor greater than 1 indicates exponential growth and a k-factor less than 1 indicates exponential decline. The k-factor in this context is itself a product of the rates of distribution and infection for an app (or virus). "Distribution" measures how many people, on average, a host will make contact with while still infectious and "infection" measures how likely a person is, on average, to also become infected after contact with a viral host.