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By Root 270 0

fever that swept across Europe every few decades. And ever since, influenza has raged through our species. In 1918, a particularly virulent outbreak of the flu killed an estimated fifty million people. Even in years without an epidemic, influenza takes a brutal toll. Each winter, thirty-six thousand people die of the flu in the United States alone; somewhere between a quarter million and a half million people die worldwide. Today scientists know that influenza is not the work of the heavens, but of a microscopic virus. Like cold-causing rhinoviruses, influenza viruses manage to wreak their harm with just ten genes. They spread in the droplets sick people release with their coughs, sneezes, and running noses. A new victim may accidentally breathe in a virus-laden droplet or pick it up on a doorknob and then bring now-contaminated fingers in contact with their mouth. Once a flu virus gets into the nose or throat, it can latch onto a cell lining the airway and slip inside. As flu viruses spread from cell to cell in the lining of the airway, they leave destruction in their wake. The mucus and cells lining the airway get destroyed, as if the flu viruses were a lawn mower cutting grass.

In healthy people, the immune system is able to launch a counterattack in a matter of days. In such cases, the flu causes a wave of aches, fevers, and fatigue, but the worst of it is over within a week. In a small fraction of its victims, the flu virus opens the way for more serious infections. Normally, the top layer of cells serves as a barrier against a wide array of pathogens. The pathogens get trapped in the mucus, and the cells snag them with hairs, swiftly notifying the immune system of intruders. Once the influenza lawnmower has cut away that protective layer, pathogens can slip in and cause dangerous lung infections, some of which can be fatal.

For a virus that has caused so much death in the past, and which continues to claim so many victims each year, influenza virus remains surprisingly mysterious. Seasonal flu is most dangerous for people with weak immune systems that can’t keep the virus in check—particularly young children and the elderly. But in flu pandemics, like the 1918 outbreak, people with strong immune systems proved to be particularly vulnerable. Scientists don’t know why the flu switches targets this way. One theory holds that certain strains of the flu provoke the immune system to respond so aggressively that it ends up devastating the host instead of wiping out the virus. But some scientists doubt this explanation and think the true answer lies elsewhere. Scientists also don’t know when influenza viruses first started making people sick. There certainly are historical records of outbreaks of deadly fevers going back thousands of years, but it’s impossible to know whether influenza viruses caused them, or another species of virus with similar symptoms.

Amidst all the mysteries of the flu, the origin of the virus is clear. It came from birds. Birds carry all known strains of human influenza viruses, along with a vast diversity of other flu viruses that don’t infect humans. Many birds carry the flu without getting sick. Rather than infecting their airways, flu viruses typically infect the guts of birds; the viruses are then shed in bird droppings. Healthy birds become infected by ingesting virus-laden water.

Sometimes strains of bird flu jump the species barrier and become human viruses. But for every successful transition, there are probably many failed crossings. Bird flu viruses are well adapted to infecting their avian hosts and reproducing quickly inside them. Those adaptations make them ill-suited to spreading among humans. Starting in 2005, for example, a strain of flu from birds called H5N1 began to sicken hundreds of people in Southeast Asia. It is much deadlier than ordinary strains of seasonal flu, and so public health workers have been tracking it closely and taking measures to halt its spread. For now, at least, H5N1 can only move from a bird to a human; it cannot move from one human to another.