Virality is a mathematical model that assumes something like the following*:
In a certain time period, new funders that make a donation will generate additional funders for the campaign, which then make donations the following period.
The average number of new funders in the next period (as a result of the activity of a single funder in the current period) is the definition of the famous viral coefficient or K factor.
But you don't need a math degree to create these
loops...
Companies such as Hotmail, Dropbox and LinkedIn have all relied
on viral loops to accelerate their growth.
That's why LinkedIn lets you email everyone in your contact list. The easier it is to share, the more likely the thing will be shared with more people, increasing the viral coefficient.
However, you can use the same techniques that these tech companies use to create your own viral loops. You just need to build these 3 steps into every funder interaction: