that causes epidemics (widespread outbreaks of
disease) every year. In the United States each year
about 20 percent of the population becomes ill
with influenza (about 60 million people), and
30,000 to 40,000 people die as a result. Occasion-
ally influenza occurs in a pandemic, in which peo-
ple worldwide become ill. The most significant
influenza pandemic in modern times was the
Spanish influenza pandemic of 1918, which sick-
ened 40 percent of the world population and
caused more than 20 million deaths. Other pan-
demics occurred in 1957 (Asian influenza) and
1968 (Hong Kong influenza). Outbreaks of Avian
flu created concern among public health officials
in the early 2000s but containment efforts pre-
vailed and limited the numbers of people who
became ill.
Influenza Virus Strains: Moving Targets
Viruses cause influenza. There are three types of
influenza viruses: influenza A, influenza B, and
influenza C. Influenza A and B are responsible for
most cases of illness; influenza C infections are
generally mild and not so easily spread from one
person to another as are A and B influenza
viruses. Every year the strains of the influenza
VIRUSresponsible for causing illness are slightly dif-
ferent from the strains that caused infection the
previous year (epidemiologists call this “drift”).
These changes help the virus survive. Having an
influenza infection confers IMMUNITYagainst the
strain of virus that caused it. Because the strains
vary each year, however, this immunity has value
only for the duration of the flu season in which
the virus strain is active (though a small amount
of resistance may carry over to similar strains).
Epidemiologists and researchers attempt to predict
which strains will emerge each year, and base
annual influenza vaccines on those strains.
Occasionally the influenza virus makes a dra-
matic alteration, a phenomenon epidemiologists
call “shift.” These are the influenza viruses capable
of causing pandemic, or worldwide, infection
because no immunity exists against them. Health
organizations around the world have monitoring
systems in place to detect these viruses and
respond before pandemic infection develops.
Because bird populations serve as reservoirs for
influenza viruses that can also infect humans,
health officials closely monitor avian influenza
infections among birds. Avian influenza outbreaks
among domesticated birds in parts of Asia in the
late 1990s and early 2000s caused alarm for the
potential of a pandemic, though containment
responses were effective in confining the out-
breaks. The ease with which people travel around
the world creates considerable challenge for con-
taining outbreaks.Influenza Vaccination
Vaccines provide immunity by stimulating the
IMMUNE SYSTEMenough to produce antibodies to
fight the virus at its next attempt to enter the
body but not enough to cause illness. The result-
ing immunity is effective against only the specific
strain of virus. Two kinds of influenza vaccines are
available in the United States:- The conventional flu shot contains inactivated
(killed) influenza virus, which, when injected
into the body, cause the immune system to
respond. The first killed-virus influenza VACCINE
became available in the United States in 1945.
Anyone older than six months of age can
receive the flu shot. - The live attenuated vaccine, which comes in
the form of a nasal spray, contains live but
weakened influenza virus genetically altered so
it cannot cause illness (it is unable to survive at
body temperature). The weakened virus enters
the bloodstream via the mucous membranes of
the nasal passages. Like the inactivated
influenza injected vaccine, the live attenuated
vaccine activates an immune system response
to produce antibodies. The live attenuated
influenza vaccine became available in the
United States in 2003. Only people between the
ages of 5 and 49 who are healthy can receive
the live attenuated virus.
Health experts recommend getting the
influenza vaccine in October or November, as the
flu season in the United States typically runs
December to March each year. Though everyone
can benefit from vaccination, certain groups of
people are at high risk for infection. They includeinfluenza prevention 35