Flu’s little helper

Influenza is a nasty bug on its own, but as doctors have long known, the other infections flu patients pick up while sick are often what kill them. This was the pattern during last year’s swine flu pandemic: some fatalities were caused by flu alone, but more were caused by the combination of influenza and bacterial pneumonia.

Immunobiologist Ruslan Medzhitov wanted to find out why flu patients so often succumb to common bacteria that our immune systems can usually dispatch. His lab studied flu infection in mice. When he injected mice with a common bacterium that “you get every time you eat a chicken sandwich,” mice that already had the flu got much sicker than healthy mice. By measuring several key immune system markers, Medzhitov found that the immune systems of flu-infected mice were significantly compromised, and that the effect lasted for about five days. Further research revealed the cause: flu infection appeared to trigger production of an immune-suppressing hormone.

But if a disabled immune system made the mice susceptible to bacterial infections, a fully active one had worse effects. When Medzhitov’s team repeated their experiments with surgically altered mice unable to produce the hormone, mice died at even higher rates. Unchecked, their immune response to flu was so aggressive that it wreaked havoc on their lungs, destroying critical tissue. Medzhitov concluded that the hormone’s immune-dampening effect, though dangerous, is essential for surviving influenza.

The study, published in Cell Host and Microbe, doesn’t provide any medical advice. But it does confirm the folk wisdom that when you get the flu, it’s good to stay home for a few days and keep yourself safe from other germs. “Grandmother was right,” says Medzhitov.