This SIR model [of spread of disease ]shows [applied to markets] that with a big enough market, you can go viral even with a small β (sharing rate) so long as your γ (churn rate) is also small. It also shows that the effects of churn cannot be ignored, even very early in viral growth.
S – The number of people susceptible to the disease (potential customers)
I – The number of people who are infected with the disease (current customers)
R – The number of people who have recovered from the disease (former customers)
See on spinnakr.com