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Learn More
In Learn More the National Geographic magazine team shares some of its best sources and other information to expand your knowledge of our featured subjects. Special thanks to the Research Division.

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Did You Know?Did You Know?

There are three types of influenza–A, B, and C. Type C viruses trigger mild respiratory illness or, sometimes, no symptoms at all. Type A and B viruses, on the other hand, can cause epidemics in humans. They result in an average of 36,000 deaths and 114,000 hospitalizations in the U.S. each year. While type B viruses are normally exclusive to humans, type A viruses are also found in birds, pigs, horses, seals, whales, and other animals.
Type A flu viruses, H5N1 for example, are named for two proteins on their surface. The H stands for hemagglutinin, which helps a virus break into the cell. The N stands for neuraminidase, which helps it break out. These proteins also act as antigens. If the body recognizes them, its immune system can mount a defense.
All type A viruses attack human cells in the same way. Once the virus enters the body, hemagglutinin binds the virus to the cell surface. Then the cell surface folds in around the virus. The virus sinks into the cell until it is completely engulfed. Because the cell membrane wraps around the virus, it creates a bubble-like compartment called an endosome. A change in pH in this compartment changes the structure of the hemagglutinin and allows the viral contents to move into the cell's inner fluid, or cytoplasm.
Once there, the virus has free reign. The virus's genetic material, which is split into eight different segments, is copied in the nucleus. After being copied, the segments return to the cytoplasm, and with the help of neuraminidase, can reassemble, leave the cell, and attack new cells. However, since each of the eight segments is copied separately, if two different viral strains infect the same cell, then their segments can mix and match, ultimately forming new viruses.
—Elizabeth Quill


Related Links

World Health Organization
This WHO website monitors the spread of avian flu to humans, provides information on control measures, and discusses why scientists are so concerned about a future pandemic.
Centers for Disease Control and Prevention
Log on to this site to get a wealth of information on avian flu including details on the H5N1 virus, the antiviral drugs available to combat it, the plans formulated to prepare and respond to a pandemic, and travel advisories for countries experiencing avian-flu outbreaks.
Program for Monitoring Emerging Diseases
This Internet-based reporting system offers daily updates of infectious disease outbreaks around the world.
United Nations Food and Agriculture Organization
Come here to learn some of the strategies being used to insure food safety and animal good health in areas facing an avian-flu crisis.



Crosby, Alfred W. America's Forgotten Pandemic. Cambridge University Press, 2003.
Drexler, Madeline. Secret Agents: The Menace of Emerging Infections. Joseph Henry Press, 2002.
Fauci, Anthony S. "Race Against Time." Nature (May 25, 2005), 423-4.
Osterholm, Michael T. "Preparing for the Next Pandemic." New England Journal of Medicine (July 7, 2005), 1839-42.
Specter, Michael. "Nature's Bioterrorist." New Yorker (February 28, 2005), 50-61.
Webster, Robert, and Diane Hulse. "Controlling Avian Flu at the Source." Nature (May 25, 2005), 415-6.
World Health Organization. "Avian Influenza: Assessing the Pandemic Threat." January 2005. Available online at www.who.int/csr/disease/influenza/WHO_CDS_2005_29/en/.


NGS Resources

Jaret, Peter. Impact: Dispatches From the Front Lines of Global Health. National Geographic Books, 2003.
Weiss, Rick. "War on Disease." National Geographic (February 2002), 4-31.
Clynes, Tom. "Dangerous Medicine: On the Front Line of the Ebola Epidemic." National Geographic Adventure (May/June 2001), 100-10, 138-42.
Jaret, Peter. "Viruses: On the Edge of Life, On the Edge of Death." National Geographic (July 1994), 58-91.
Jaret, Peter. "The Disease Detectives: Stalking the World's Epidemics." National Geographic (January 1991), 114-40.
Tran Van Dinh, "Hue: My City, Myself." National Geographic (November 1989), 594-603.
Jaret, Peter. "Our Immune System: The Wars Within." National Geographic (June 1986), 702-35.

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