One of the difficult things to accept about this influenza episode is the uncertainty that surrounds many of the official pronouncements about the outbreak.  This uncertainty about “possible” pandemics or “probable” sources of contagion stems from two different sources.  First, there is still much we do not know about this particular virus and its hosts (more on this later).  But even if we knew everything there is to know– which we will not– we need to keep in mind that epidemiology is neither a deterministic, nor an inherently predictive discipline.  The statements that epidemiology is equipped to make are inherently probablistic.  We can predict that the chances of an outbreak are increasing (or decreasing) under certain circumstances, but we cannot guarantee that a particular strain will spread at a given location by a certain time.

The idea that all science deals in absolute truths and certainties is a profound misunderstanding of the nature of our work.  Many branches of science deal with problems where uncertainty is fundamental to the process or objects of study.  Epidemiology is a prime example: so many different processes determine the dynamics of an infection that we cannot be certain of any single outcome.

When the WHO decides to raise the pandemic alert to Phase 5, they are not predicting that a global influenza pandemic will occur.  Rather, in their judgement, the conditions on the ground make the possibility of a pandemic far more likely.  But prediction is not guarantee, and we cannot look to the science of complex phenomena for simple certainty.