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# Focus on the Flu

## Research: Flu Primer

### What We Know About the Flu Virus

A flu virus is roughly round, but it can also be elongated or
irregularly shaped. Inside are eight segments of single-strand RNA
containing the genetic instructions for making new copies of the virus.
Flu\'s most striking feature is a layer of spikes projecting from its
surface. There are two different types of spikes: one is the protein
hemagglutinin (HA), which allows the virus to \"stick\" to a cell and
initiate infection, the other is a protein called neuraminidase (NA),
which enables newly formed viruses to exit the host cell.

### Virus A, B, C

Influenza viruses are classified as type A, B, or C based upon their
protein composition. Type A viruses are found in many kinds of animals,
including ducks, chickens, pigs, whales, and also in humans. The type B
virus widely circulates in humans. Type C has been found in humans,
pigs, and dogs and causes mild respiratory infections, but does not
spark epidemics.![](media/image2.jpeg){width="2.0819444444444444in"
height="1.7909722222222222in"}

Type A influenza is the most frightening of the three. It is believed
responsible for the global outbreaks of 1918, 1957 and 1968. Type A
viruses are subdivided into groups based on two surface proteins, HA and
NA. Scientists have characterized 16 HA subtypes and 9 NA subtypes.

### Naming Viral Strains

Type A subtypes are classified by a naming system that includes the
place the strain was first found, a lab identification number, the year
of discovery, and, in parentheses, the type of HA and NA it possesses,
for example, A/Hong Kong/156/97 (H5N1). If the virus infects non-humans,
the host species is included before the geographical site, as in
A/Chicken/Hong Kong/G9/97 (H9N2). There are no type B or C subtypes.

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### Where Influenza Comes From

In nature, the flu virus is found in wild aquatic birds such as ducks
and shore birds. It has persisted in these birds for millions of years
and does not typically harm them. But the frequently mutating flu
viruses can readily jump the species barrier from wild birds to
domesticated ducks and then to chickens. From there, the next stop in
the infectious chain is often pigs.

Pigs can be infected by both bird (avian) influenza and the form of
influenza that infects humans. In a setting such as a farm where
chickens, humans and pigs live in close proximity, pigs act as an
influenza virus mixing bowl. If a pig is infected with avian and human
flu simultaneously, the two types of virus may exchange genes. Such a
\"reassorted\" flu virus can sometimes spread from pigs to people.

Depending on the precise assortment of bird-type flu proteins that make
it into the human population, the flu may be more or less severe.

In 1997, for the first time, scientists found that bird influenza
skipped the pig step and infected humans directly. Alarmed health
officials feared a worldwide epidemic (a pandemic). But, fortunately,
the virus could not pass between people and thus did not spark an
epidemic. Scientists speculate that chickens may now also have the
receptor used by human-type viruses.

### Drifting and Shifting

Influenza virus is one of the most changeable of viruses. These genetic
changes may be small and continuous or large and abrupt.

Small, continuous changes happen in type A and type B influenza as the
virus makes copies of itself. The process is called antigenic drift. The
drifting is frequent enough to make the new strain of virus often
unrecognizable to the human immune system. For this reason, a new flu
vaccine must be produced each year to combat that year\'s prevalent
strains.

Type A influenza also undergoes infrequent and sudden changes, called
antigenic shift. Antigenic shift occurs when two different flu strains
infect the same cell and exchange genetic material. The novel assortment
of HA or NA proteins in a shifted virus creates a new influenza A
subtype. Because people have little or no immunity to such a new
subtype, their appearance tends to coincide with very severe flu
epidemics or pandemics.