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 happ… - Sanjay Gupta

Panˈdemik/: pan means "all"; demic (or demographic) means "people." It is well-named, because pandemic flu spreads easily throughout the world. Unlike seasonal flu, pandemics occur when a completely new or emerges. This sort of virus can emerge directly from animal reservoirs or be the result of a dramatic series of mutations -- so-called events -- in previously circulating viruses. In either case, the result is something mankind has never seen before: a that can spread easily from person to defenseless person, our immune systems never primed to launch any sort of defense.

flu is apolitical and does not discriminate between rich and poor. Geographical boundaries are meaningless, and it can circle the globe within hours. In terms of potential impact on mankind, the only thing that comes close is climate change. And, like climate change, pandemic flu is so vast, it can be challenging to wrap your head around it.

I was in my teens when our family faced a medical crisis. My grandfather, with whom I was very close, had a stroke and landed in the hospital. Sitting anxiously at his bedside, I watched nurses come and go, checking his vitals and looking at the monitors attached to his body. I remember sitting there wondering what could I do to make him feel better—to bring back the warm, thoughtful man I knew.