New Orleans

AS the swine flu threatens to become the next pandemic, the biggest questions are whether its transmission from human to human will be sustained and, if so, how virulent it might become. But even if this virus were to peter out soon, there is a strong possibility it would only go underground, quietly continuing to infect some people while becoming better adapted to humans, and then explode around the world.

What happens next is chiefly up to the virus. But it is up to us to create a vaccine as quickly as possible.

Influenza viruses are unpredictable because they are able to mutate so rapidly. That capacity enables them to jump easily from species to species, infecting not only pigs and people but also horses, seals, cats, dogs, tigers and so on. An avian virus responsible for the 1918 pandemic jumped first from birds to humans, then from humans to swine (as well as other animals). Now, and not for the first time, pigs have given a virus back to humans.

Image Credit... John Hersey

Mutability makes even existing, well-known flu viruses unpredictable. A new virus, formed by a combination of several existing ones as this virus is, is even less predictable. After jumping to a new host, influenza can become more or less virulent — in fact, different offshoots could go in opposite directions — before a relatively stable new virus emerges.