It’s been a rough flu season this winter in the United States and Europe, but it could be worse. A lot worse. The flu viruses that are making us sick go by names like H1N1 and H3N2, referring to the kinds of proteins that stud their surface. There’s another sort of flu lurking in other parts of the world, like Egypt, India, and Cambodia, known as H5N1. Since 2003, 615 people have come down with H5N1, and, as of Feburary 1, 364 of them had died. In January alone, 5 people in Cambodia were diagnosed with H5N1. Four of them died.
There’s a lot of debate about precisely how bad H5N1 is. It’s possible that a lot of people are getting sick with H5N1 without making it onto the official records. They’re crawling off into bed for a week, recuperating, and then getting on with their lives. So the 59 percent death rate you get from the official numbers (what’s known as the case-fatality rate) may be a serious overestimate. Still, even if the true rate was only half as high, H5N1 would not be a virus you’d want to cross paths with. The most famous flu outbreak of all, the so-called Spanish flu of 1918, is estimated to have killed 50 to 100 million people worldwide. But it infected billions, with a death rate of roughly 2 percent. If H5N1 could somehow take off and become a global pandemic, it would become an unparalleled catastrophe even if its official 59 percent death rate was chopped down by a factor of ten.
Right now, H5N1 does not have what it takes to race around the world. It might someday, although nobody can say for sure what the odds are. And, as a team of scientists now report in the Philosophical Transactions of the Royal Society, H5N1 would continue to evolve as it spread. Its ability to spread would evolve, along with its ability to kill. The scientists can’t look into their crystal ball and say for sure how many people would die. But they can say this: what we do when and if we face an H5N1 pandemic could alter the evolution of the virus itself. And thousands of lives could be saved or lost as a result.
All of the flu strains that make us humans sick get their start in birds. Our feathered friends harbor a huge diversity of flu viruses in their guts, which they shed in their droppings. Most bird flu strains are fairly harmless to the birds themselves–they don’t overhwhelm their immune system, in other words. Most of them can’t infect animals other than birds. But on rare occasion, a strain of bird flu evolves that can also latch onto cells in another species, such as a seal or a human.
That’s what H5N1 has done. In at least some of the people who are exposed to it–from their chickens or from wild waterfowl–H5N1 can latch onto cells in the airway and invade. What H5N1 can’t do is spread from one person to another. There may be a number of reasons for this. A successful human flu virus needs to find a way to enter human cells quickly, it needs to hijack human biology to make new viruses, it needs to escape human immune attacks, and it needs to be able to spread efficiently from one person to the next.
Exactly what it would take for H5N1 to become a true human flu got two teams of scientists into hot water for back in 2011. They introduced mutations into H5N1 and then infected ferrets with it. (Ferrets get sick from the flu a lot like we do, so they’re pretty good models to study.) They then swabbed viruses from the noses o... http://phenomena.nationalgeographic.com/2013/02/07/the-future-evolution-of-bird-flu/
