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Part 2: What is influenza — and who gets it?

To best understand the material in this web article, read the sections in our textbook that explain how the immune system works. (Dragonfly book Sections 40-1 and 40-2; Elephant book sections 45-1 and 45-2.)

Influenza (flu)
is caused by a virus that attacks the nose, throat, and lungs. Most of the time, for most people, influenza is a moderately serious illness that lasts for less than two weeks. But sometimes influenza can be much more serious -- both to individual patients and to global human society.

This false-color electron micrograph shows the generally spherical shape of the type A influenza virus family. Right: The virus has an 8-stranded RNA genome that codes for 10 proteins. Different strains of the virus are classified according to variations in two of those proteins — hemagglutinin and neuraminidase, which protrude form the surface of the viral envelope.(micrograph courtesy of Gopal Murti, St. Jude Children's Research Hospital. Illustration by Emma Skurnick.)



The Flu is a serious threat in the US nearly every year


Check out the website for the American Experience episode dealing with the 1918 influenza epidemic.



This influenza ward at Camp Fuston, Kansas, is shown filled with military influenza patients close to the end of WW I.

It may be hard to imagine a world devastated or thrown into panic by the flu. But several times during the 20th century, unusually deadly influenza strains caused epidemics that killed hundreds of thousands of people. In 1918 for example, while our country mobilized for World War I, an influenza epidemic swept across the country, killing 675,000 Americans and somewhere between 20 and 50 million people worldwide — more than all the wars of the 20th century put together . This epidemic is seldom mentioned, and most Americans seem to have forgotten about it altogether. To get an idea of how serious it actually was, read this letter from a doctor stationed at Fort Devens in Massachusetts.

Fighting influenza would be a lot simpler if the virus that caused it could only infect our species. But in fact, influenza viruses also infect other animals. Does that surprise you? Most of us, if we think about this subject at all, usually think of "our" diseases as separate from illnesses of other species. Sure, the threat of bioterrorism reminded those of us who don't live in cattle country that both livestock and humans can get anthrax. And we've learned that ticks jumping from deer to mice to humans can sometimes carry Lyme disease. But most of the time, we act as though those cases are exceptions.

They aren't. Many infectious diseases affect a wide range of host species. You may or may not know that after researchers discovered the Human Immunodeficiency Virus (HIV), they discovered very similar Simian Immunodeficiency Viruses (SIV's) that infect chimpanzees and other primates. Then they found Feline Immunodeficiency Viruses (FIV's) in domestic and wild cats (including lions and cheetahs), and several other groups of viruses in other groups of mammals. (See M&L web feature (to come) "What do AIDS, Flu, SARS, Lyme Disease, and West Nile Virus have in common?")

That's true for influenza too. Different strains of influenza viruses attack many other species, including ducks, chickens, turkeys, pigs, horses, and ferrets. Some virus strains can jump from one host species to another — especially if the host species are closely related. Several strains, for example, readily infect humans, pigs, and ferrets. Remember this point, because it is a vital clue to the way serious flu epidemics can arise.

Poultry in Japan have also been hard hit by the Avian flu this year

Some influenza strains, however, are usually restricted to one group of closely-related host species.  Avian (bird) flu strains, for example, can often infect several species of birds but seldom, if ever, cause disease in  humans. And here's the rub: because avian strains do not normally infect our species, they evolve separately from human strains. As a result, bird viruses often carry different genes that code for proteins on the outer virus coat that differ significantly from the coat proteins of human virus strains. This, as you will see shortly, is what makes these strains potentially dangerous to us.

Index of this Web Article:

• Introduction — What's up with the Flu?
• What is influenza — and who gets it?
Why can we get the flu again and again? And why can't we develop a one-time flu vaccine?
• How are flu vaccines made?
• Why are flu vaccines sometimes not effective?
• Why are researchers so worried about this new bird flu?

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