How does the flu work?
Wed, Dec 02 2009 at 8:00 AM EST
Read more: SWINE FLU, VIRUSES & DISEASES
A virus known as H1N1 has blanketed the Earth in less than a year, and as cold weather creeps across the Northern Hemisphere, the new flu may just be getting warmed up. While it has been less deadly than many feared after its initial outbreak in April, more than 7,820 people have nonetheless been killed, and the pandemic is already making this year's normal flu season much less normal.The hazy origins and sudden explosion of H1N1, also known as "swine flu," give the disease a menacing mystique that's frightened and fascinated millions. Although much of its evolution remains a mystery, the virus itself isn't as alien as it might seem — until, however, you consider what H1N1 and all its relatives actually do once they get inside someone's body.
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DiggSwine flu, bird flu, human flu and dog flu are all different versions of the same familiar foe. Influenza can be surprisingly crafty for a brainless bundle of genes, often evolving new traits spontaneously — including the ability to become deadlier and jump between species.
Those two traits in particular make the flu especially outbreak-prone, helping it spur pandemics in 1968, 1957 and most horribly in 1918. That year's "Spanish flu" infected up to 40 percent of the human population, killing more than 20 million people worldwide.
Underneath its threats to civilization, though, the flu operates like most other viruses. It can do almost nothing on its own, twisting helplessly in the wind until someone happens to inhale it. It contains only eight genes — humans have at least 20,000 — but that's enough to jump-start a vicious cycle that turns host cells into unwitting slaves, churning out more and more viruses that go on to infect more and more cells.
The flu virus has to make major adaptations for each species it infects; it attacks the digestive systems of birds, for example, but targets our respiratory systems. Pigs are known as incubators of pandemics because they can catch both mammal and bird flus, even simultaneously — since the viruses are switching genes all over the place, they can take on properties of each other, namely the ability to infect another host species.
Among humans, the flu spreads mostly in respiratory droplets from coughs or sneezes, which can project impressively long distances. An infected person is contagious one day before showing symptoms and up to five days afterward. The disease is typically worst in winter, presumably because it stays aloft longer in the dry air.
In like phlegmOne of any virus's greatest achievements is actually getting inside a cell. The cell membrane normally keeps out intruders, but successful viruses are studded with special proteins (see illustration at right) that fit into receptors on the membrane's surface, essentially picking the lock.
Once inside, the virus is suddenly immersed in the cytoplasm, a liquid region filling much of a cell's interior. It floats inward to the nucleus, the center of which is full of our genetic material and plugging away making more cells. By this point the flu has already beaten most of the cell's defenses, so it just waltzes up to the nucleus and releases its own genetic material — 10 strands of ribonucleic acid, aka RNA, in this case.
Under the influenza
The nucleus is the nerve center of a cell, but calling it the brain might be giving it a little too much credit. The flu, and other viruses, are able to simply hand the nucleus a new set of blueprints and trick it into making new viruses instead of new cells.
In addition to allowing the virus to reproduce and spread in the short term, this mess of gene-swapping is one of the two ways viruses evolve. The other way is called antigenic drift — the subtle, random mutations in the virus's lock-picking proteins, or "antigens," that allow it to evade our immune responses that we adapted for previous generations of the virus.
In antigenic shift, however, much more drastic changes can take place (click on the flow chart at right for an illustrated explanation). This usually requires the host to be simultaneously infected with two viruses, whose genes get mixed up in the nuclei of host cells besieged by both strains. The cell then produces hybrid viral strains that may have mashed-up components of each original strain. That's how a bird or swine flu, for example, can make the jump to infect humans, and it's how all three combined to spark the recent H1N1 outbreak. (Canine flu has yet to infect humans, but the CDC warns that it's always possible; it already jumped from horses to dogs.)Once the hijacked nucleus starts pumping out copies of the flu's RNA, they're processed into full-grown viruses in the cytoplasm. Some of these glide out of the cell as easily as they came in, but cells also often fill up with the viruses they're producing, break open and then die, which can be destructive on a large enough scale. As the new viruses stream out of sickened and ruptured cells, they go on to infect a growing number of other cells until, ideally, the body's immune system can figure out a way to stop them.
The spread of swine flu
More than 622,000 cases of the pandemic H1N1 flu have been confirmed globally, according to the World Health Organization, but since many countries have stopped counting individual cases, the actual number of infections is likely much higher. The disease has spread to at least 207 countries, killing roughly 7,820 people around the world.
The first U.S. case of swine flu was confirmed on April 15, and by mid-June the virus had spread to all 50 states (use the map widget above to see its spread over time). There have now been an estimated 98,000 hospitalizations and 3,900 deaths due to H1N1 in the United States, according to the CDC.
By comparison, the seasonal flu normally hospitalizes some 200,000 Americans every year and kills 36,000. While that version of the virus tends to hit elderly folks hardest, however, H1N1 has a knack for attacking a younger demographic: More than half of U.S. cases and about 75 percent of U.S. deaths have been in people between the ages of 18 and 64, compared with roughly 9 percent of cases and 11 percent of deaths in Americans older than 65. Scientists believe many older people may have developed an immunity to the new virus, thanks to their exposure to previous pandemic strains in 1968, 1957 and even 1918.
H1N1 has proven its skill in stampeding through human populations, growing into a pandemic within a few weeks of its debut on the public radar last spring. And it may have a few more tricks up its sleeve, too — scientists in Norway recently isolated a mutated version of the H1N1 virus, for example, which they warned is possibly even deadlier than the original. Despite its predatory prowess, however, H1N1 is still classified as a "mild" flu strain, partly because it doesn't invade our airways as deeply as some other types. It tends to focus on humans' upper respiratory tracts, where it can reproduce more quickly but where its symptoms are also less severe than if it delved deeper into the lungs.
Flu is always an erratic, unpredictable virus — especially when multiple strains co-exist in dense populations of hosts — but the CDC has mustered enough certainty to predict that H1N1 outbreaks will ultimately make the 2009-'10 flu season worse than usual. The U.S. flu season's timing varies from year to year, but it can begin as early as October and usually peaks in February.
For more information and the latest news about swine flu, or about viruses and diseases in general, see their pages in MNN's Eco-Glossary. And don't miss our photo gallery of 10 flu-fighting foods, in addition to these other popular H1N1 links on MNN:
See the following links for more from the CDC about how H1N1 spreads, how to avoid it, what's up with vaccines and how to take care of a sick person in your home. For another look at how H1N1 evolved and how it works, check out the video below, produced by the U.K.'s Channel 4:
And for more government info about H1N1 and the flu's species-hopping issues, see the links below:
- Flu.gov: Swine Flu (H1N1)
- Flu.gov: Bird Flu (H5N1)
- Flu.gov: Animal and Bird Issues
- CDC: H1N1 Background
- CDC: Transmission of Influenza Viruses from Animals to People
Editor's Note: This article has been updated from its original version, which appeared on May 7, 2009.
Flu photo and illustrations: U.S. National Institutes of Health
Flu map: CDC
MNN homepage photo: ArtisticCaptures/iStockPhoto
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Comments(11)
Posted By Anonymous - Wed, Dec 02 2009 at 10:33 AM ESTSwine flu catching in winter
I would like to know if the swine flu is going to catch again this winter.. do I need to take any vaccine..
Posted By Skye B. - Tue, Oct 27 2009 at 2:58 PM ESTTwo Videos Will Help Everyone Make Sense Of It
TWO Videos Everyone Needs To Watch:
How To Fight The H1N1 Virus ("#Swine Flu") http://bit.ly/4D32We
How To Prevent The Swine Flu http://bit.ly/2WIlIM
Posted By Inman Aligner birmingham - Sun, Nov 22 2009 at 12:17 PM ESTHow many affected
How many affected inUK
Posted By smith123 - Thu, Oct 08 2009 at 2:49 PM ESTflu
But why is it called swine flu? wondering the same thing?
Posted By Anonymous - Wed, Sep 30 2009 at 5:09 PM ESTH1N1 Can Be Serious
Good Article. Yes, the swine flu can be serious. My 19 year old daughter was hospitalized for five days this month. Individuals with compromised immune systems (diabetes, asthma, etc.) can get very sick. I think that getting Tamiflu within 48 hours of flu symptoms helped.
Posted By hsr0601 - Tue, Sep 29 2009 at 4:45 AM ESTSwine flu vaccines are thought to be safe and effective
1. Swine flu vaccines are thought to be safe and effective as the initial symptom is mild.
2. Folks need to stay vigilant on refraining form the in-take of pork, just in case of the mutation.
(( Genes included in the new swine flu have been circulating undetected in pigs for at least a decade, according to researchers who have sequenced the genomes of more than 50 samples of the virus. The findings suggest that in the future, pig populations will need to be monitored more.... More
Posted By Gorax - Wed, Dec 02 2009 at 10:35 AM ESTOh! I know! I know!
How the flu works:
1 - Bring flu to everyone's attention
2 - Exaggerate severity of said flu
3 - Praise big pharma's awesome vaccines
4 - Profit!
For those of us not swept away in the wave of hysteria, there's a healthy diet and an immune system.
Posted By Anonymous - Thu, Sep 17 2009 at 9:02 AM ESTSwine flu
Nice article. But why is it called swine flu ?
is the Swine flu symptoms> listed correct?
Posted By GreenErinF - Thu, May 14 2009 at 11:12 AM ESTFun animations
Thanks for this! The animations are easy to understand and it helped me to see how the flu virus makes copies of itself and then ends up infecting other cells. It's like a fun science lesson. :)
Companion Literature
See the following You tube video snippet for a companion literature.
http://www.youtube.com/watch?v=y77oExTDw8I&NR=1