Influenza WORLD OF MICROBIOLOGY AND IMMUNOLOGY312•
oviruses. Infection with these viruses leads to a self-limiting
illness usually characterized by fever, muscle aches, fatigue,
and upper respiratory infection and inflammation. Children
and young adults usually recover from influenza within 3–7
days with no complications; however, in older adults, espe-
cially those over 65 with underlying conditions such as heart
disease or lung illnesses, influenza can be deadly. Most of
the hospitalizations and deaths from influenza occur in this
age group. Although an influenza vaccineis available, it
does not confer complete protection against all strains of
influenza viruses.
Like all viruses, orthomyxoviruses cause illness by enter-
ing host cells and replicating within them. The new viruses then
burst from the host cell and infect other cells.
Orthomyxoviruses are sphere-shaped viruses that contain
ribonucleic acid(RNA). The viruses use this RNA as a blue-print
for replication within host cells. The outer envelope of an
orthomyxovirus is studded with protein spikes that help the
virus invade host cells. Two different types of spikes are present
on the virus’s outer envelope. One type, composed of hemag-
glutininprotein (HA), fuses with the host cell membrane, allow-
ing the virus particle to enter the cell. The other type of spike,
composed of the protein neuraminidase (NA), helps the newly
formed virus particles to bud out from the host cell membrane.
The only way a virus can be neutralized and stopped is
through the body’s immune response. At the present time, no
cure or treatment is available that completely destroys viruses
within the body. The HA spikes and proteins in the orthomyx-
ovirus envelope stimulate the production of antibodies,
immune proteins that mark infected cells for destruction by
other immune cells. In a healthy person, it takes about three
days for antibodies to be formed against an invading virus.
People with impaired immune function (such as people with
Acquired Immune Deficiency Syndrome, the elderly, or peo-
ple with underlying conditions) may not be able to mount an
effective immune response to the influenza virus. Therefore,
these people may develop serious complications, such as
pneumonia, that may lead to hospitalization or death.Three types of orthomyxoviruses cause illness in
humans and animals: types A, B, and C. Type A causes epi-
demic influenza, in which large numbers of people become
infected during a short period of time. Flu epidemicscaused
by Type A orthomyxoviruses include the worldwide outbreaks
of 1918, 1957, 1968, and 1977. Type A viruses infect both
humans and animals and usually originate in the Far East,
where a large population of ducks and swine incubate the virus
and pass it to humans. The Far East also has a very large
human population that provides a fertile ground for viral repli-
cation. In 1997, a new strain of influenza A jumped from the
poultry population in Hong Kong to the human population.
H5N1, as the strain was named, was contracted through con-
tact with the feces of chicken. The illness it caused (dubbed
avian flu) was severe, and sometimes fatal. Although it was
strongly believed that humans could not get the disease from
eating properly cooked chicken, the decision was ultimately
made to destroy and bury all of the chickens in Hong Kong.
This massive effort was carried out in December 1997.
Type B influenza viruses are not as common as type
A viruses. Type B viruses cause outbreaks of influenza
about every two to four years. Type C viruses are the least
common type of influenza virus and cause sporadic and
milder infections.
The hallmark of all three kinds of influenza viruses is
that they frequently mutate. Due to the small amount of RNA
genetic material within a virus, mutation of the genetic mate-
rial is very common. The result of this frequent mutation is
that each flu virus is different, and people who have become
immune to one flu virus are not immune to other flu viruses.
The ability to mutate frequently therefore allows these viruses
to cause frequent outbreaks.
Influenza is characterized by a sudden onset of fever,
cough, and malaise. The incubation period of influenza is
short, only 1–3 days. The cells that the influenza virus target
are the cells of the upper respiratory tract, including the
sinuses, bronchi, and alveoli. The targeting of the upper respi-
ratory tract by the viruses accounts for the prominence of res-
piratory symptoms of flu. In fact, flu viruses are rarely found
outside the respiratory tract. Most of the generalized symp-
toms of flu, such as muscle aches, are probably due to toxin-
like substances produced by the virus.
Symptoms last for about 3–6 days; however, lethargy
and cough may persist for several days to weeks after a bout
with the flu. Children may have more severe symptoms due to
a lack of general immunityto influenza viruses. Children also
have smaller airways, and thus may not be as able to compen-
sate for respiratory impairment as well as adults.
The most common complication of influenza is pneu-
monia. Pneumonia may be viral or bacterial. The viral form of
pneumonia that occurs with influenza can be very severe. This
form of pneumonia has a high mortality rate. Another form of
pneumonia that is seen with influenza is a bacterial pneumo-
nia. If the respiratory system becomes severely obstructed dur-
ing influenza, bacteriamay accumulate in the lungs. This type
of pneumonia occurs 5–10 days after onset of the flu. Because
it is bacterial in origin, it can be treated with antibiotics.Microscopic view of Infuenza virus.womi_I 5/6/03 3:23 PM Page 312