Close encounters

As far back as the 1990s, scientists warned that a new form of avian influenza could spark another flu pandemic.

H5N1 hasnt made that leap yet. But with every passing year, this nefarious virus strikes more species. It regularly kills millions of wild and farmed birds, while jumping between more than two dozen mammals, including mink, sea lions, dairy cows and alpacas. Humans, too and often with deadly consequences.

For now, were just collateral damage this virus still thrives best in birds. Scientists great fear is that H5N1 eventually gains the final, crucial adaptations needed to transmit person-to-person.

But that last leap is hard to achieve, and even tougher to predict.

So how did we get here? And is it possible to stay one step ahead of this ever-evolving virus?