geometric progression. Epidemics are another example of geometric progression: when a virus spreads through a population, it doubles and doubles again, until it has (figuratively) grown from a single sheet of paper all the way to the sun in fifty steps. As human beings we have a hard time with this kind of progression, because the end result—the effect—seems far out of proportion to the cause. To appreciate the power of epidemics, we have to abandon this expectation about proportionality.
