Ah, I hadn't thought of the word that way. It isn't re-combined, as in "combined again". HIV and the flu were not the same virus in the past.

It's a genetics term and refers to a kind of mutation. When the flu virus is copying itself in the presence of a different virus that is copying itself, some of those genetic building blocks can get mixed together (this is the recombination part) so the final product is a new combination of genes. A new strain has just been created with potentially different characteristics.

The influenza virus is particularly adept at this. It's one reason why the composition of the annual flu shot needs to be tweaked all the time. This propensity for recombination is also why health authorities want to try and minimize the spread of this H1N1, even though it seems to only produce a mild case of the flu.

Many think that things have been overkill so far, but this process of recombination could allow this mild flu virus to suddenly pick up some lethality. The more people that it infects, the greater the chance that this person has another infection that H1N1 could recombine with. So you end up with a new strain that not only spreads like a seasonal flu, but now is much more lethal. Or even a strain that is much more infectious could do a lot of damage due to the sheer numbers of people infected. That scenario is what still keeps health authorities up at night thinking about H1N1.

"Why are authorities worried about this mild new strain but aren't as worried about the regular flu?" you might ask. The seasonal flu is almost always just a variation of strains that we humans have been exposed to year after year, so we tend to build up natural immunity to it over time and we have an idea of what the virus is capable of. It would take a radical mutation to really surprise us with the seasonal flu, although technically, that's possible, too.

This H1N1 strain is completely new to us to start with, so no one knew if it would produce a mild disease or whether it would tear through the population because we don't already have any immunity to it. The number of suspected deaths in Mexico in the beginning of the epidemic put everyone on high alert. We lucked out so far with H1N1.