Looking for the inverse -- not the opposite -- of "zero-sum game"

Solution 1:

Your disease example reminds me of the term chain reaction. From NOAD:

chain reaction [figurative] a series of events, each caused by the previous one : an article in one publication sets off a chain reaction in the media.

Your candle example reminds me of the expression pay it forward, an expression which isn't established enough to make it into dictionaries yet, but is enjoying growing popularity thanks to the film of the same name. The phrase actually goes back well before that book ("You don't pay love back, you pay it forward," wrote Lily Hammond in 1913), and the concept even well before that, explains one author in forward to a work entitled The Circle of a Good Deed: Pay It Forward from Concept to Philosophy to Practice.