What’s the Difference Between Seasonal Flu, Pandemic
Flu, and Avian (Bird) Flu?
Influenza, or flu, is a respiratory infection caused
by several flu viruses. Flu viruses are classified as
types A, B, and C; type A has a number of subtypes. The
flu is not the same as the
common cold, nor is it related to what is commonly
called the “stomach flu.”
Seasonal flu is the term used to refer to the flu
outbreaks that occur yearly, mainly in the late fall and
winter. It is estimated that between 5 and 20 percent of
Americans come down with the flu every flu season.
Pandemic flu refers to particularly virulent strains
of flu that spread rapidly from person to person to
create a world-wide epidemic (pandemic).
Avian (Bird) Flu
In nature, the flu virus also occurs in wild aquatic
birds such as ducks and shore birds. It does not
normally spread from birds to humans. However, pigs can
be infected by bird influenza (as well as by the form of
influenza that affects humans) and can pass on the flu
to humans. In 1997, it was discovered that a virulent
bird influenza had skipped the pig step and had infected
humans directly, causing a number of deaths in Asia.
These instances of bird flu in humans have raised
concerns that if this type of flu could at some point be
transmitted between people, a new pandemic would occur.
Thus, the term bird flu or avian flu is currently being
used to refer to a possible pandemic flu.
Overview of the Flu
The flu, like the common cold, is a respiratory
infection caused by viruses. But the flu differs in
several ways from the common cold. For example, people
with colds rarely get fevers or headaches or suffer from
the extreme exhaustion that flu viruses cause. The most
familiar aspect of the flu is the way it can "knock you
off your feet" as it sweeps through entire communities.
The Centers for Disease Control and Prevention (CDC)
estimates that 5 to 20 percent of Americans come down
with the flu during each flu season, which typically
lasts from November to March. Children are two to three
times more likely than adults to get sick with the flu,
and children frequently spread the virus to others.
Although most people recover from the illness, CDC
estimates that in the United States more than 200,000
people are hospitalized and about 36,000 people die from
the flu and its complications every year.
Seasonal Flu Outbreaks
Seasonal flu outbreaks usually begin suddenly and
occur mainly in the late fall and winter. The disease
spreads through communities, creating an epidemic.
During the epidemic, the number of cases peaks in about
3 weeks and subsides after another 3 or 4 weeks. Half of
the population of a community may be affected. Because
schools are an excellent place for flu viruses to attack
and spread, families with school-age children have more
infections than other families, with an average of
one-third of the family members infected each year.
Importance of Flu
Besides the rapid start of the outbreaks and the
large numbers of people affected, the flu is an
important disease because it can cause serious
complications. Most people who get the flu get better
within a week (although they may have a lingering cough
and tire easily for a while longer). For elderly people,
newborn babies, and people with certain chronic
illnesses, however, the flu and its complications can be
Flu Virus Characteristics
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
virus. Credit: CDC
Virus Types 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, and whales, and also 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.
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.
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 a very severe flu epidemic or pandemic.