A diagram of the structure of an influenza virus, showing eight genes inside, and the outer spikes that can be recognized by the immune system. Diagram and electron micrograph below © 1995, Linda Stannard, Department of Medical Microbiology, University of Cape Town.
Even if you're not among the 20,000 Americans flu kills in an average year, the bug's a bummer. Between 35 million and 50 million Americans get its chills, fevers, aches and coughs. The direct medical cost of flu is guesstimated at $4.6 billion annually (not counting lost work time). Many deaths are caused by the complication of pneumonia, a lung infection that can be caused by a virus or a bacterium. Others result from exacerbations to pre-existing conditions like diabetes, heart or lung disease.
To understand flu, you've got to know something about viruses. First off, they're weird. Unlike all other forms of life, they don't have cells -- they're basically chains of genetic material lumped inside a protein coat.
Viruses can multiply and evolve, but some experts think they're not really alive, since they can't reproduce without a host cell. Host cells are innocent victims that allow the virus to enter and commandeer its machinery to make new viral particles. To some observers, viruses are the ultimate guerilla fighters.
Other virologists disagree, arguing that there really is life inside that protein coat. University of Wisconsin flu expert Virginia Hinshaw, for example, says, "To have an infectious agent that has adapted to use our machinery to reproduce, to me, that's living."
At any rate, viruses have few moving parts and are so small that you need an electron microscope to see them. To be specific, influenza viruses are just 80 to 120 billionths of a meter in diameter.
Shifting into overdrive
But it's not size that makes flu virus hard to fight. It's the virus' shifty nature; its ability to evade the immune system. That shiftiness is related to two proteins on the virus' surface, called hemagglutinin and neuraminadase. (We'll give you a break and call them by their nicknames, "H" and "N".)
H and N are the surface molecules that are "recognized" by antibodies in the human immune system. By triggering an immune response, these antibodies offer protection against infection and limit disease. Handy stuff.
If H and N remained static on the viral surface, we could easily build up immunity to flu. But they don't. H and N molecules can take many forms, and the only way to get immunity, short of vaccination, is to suffer another bout of flu.
Next year, naturally, flu's H and N molecules will take another form, forcing you to pay again (through vaccination or disease) to build more immunity. [Please pass the chicken soup!]
Scientists call this gradual change in the viral coat "antigenic drift," because it creates small changes in the antigens -- the molecules that the immune system recognizes.
Other viruses do change forms through mutation (HIV comes to mind). But in terms of making minor and major changes, "influenza is the champion," says Hinshaw.
That's right, the professor said "major."
Want to hear about the really ominous changes?
