Experts say we are "due" for one. When it happens, they tell us, it will probably have a greater impact on humanity than anything else currently happening in the world. And yet, like with most people, it is probably something you haven't spent much time thinking about. After all, it is human nature to avoid being consumed by hypotheticals until they are staring us squarely in the face. Such is the case with a highly lethal flu pandemic. And when it comes, it will affect every human alive today.