What’s up with H7N9, the new avian influenza?

I have a new article up today at Slate, examining the emergent H7N9 avian influenzas, and a bit of a review of “bird flu” in general:

While we were carefully watching H5N1 in Asia and Europe, another influenza virus—2009 H1N1—appeared seemingly out of nowhere. Ultimately traced back to swine, this virus was easily spread between people, but unlike H5N1, it wasn’t any more deadly than our normal yearly influenza viruses (which, it should be noted, still kill on the order of 36,000 Americans each year). And now, while we’re still working on understanding how H5N1 and H1N1 have jumped between species, yet another influenza type has surfaced: H7N9.