It can be difficult to distinguish between the common cold
and influenza in the early stages of these infections, and can be a
symptom in children. The symptoms most reliably seen in influenza are
shown in the table to the right.
Since antiviral drugs are effective in treating influenza if
given early, it can be important to identify cases early.
Of the symptoms listed above, the combinations of fever with
cough, sore throat and/or nasal congestion can improve diagnostic
accuracy. Two decision analysis studies suggest that ''during local
outbreaks'' of influenza, the prevalence will be over 70%.
According to the CDC, rapid diagnostic tests have a
sensitivity of 70–75% and specificity of 90–95% when compared with viral
These tests may be especially useful during the influenza
season (prevalence=25%) but in the absence of a local outbreak, or
peri-influenza season (prevalence=10%).
The amount of virus shed appears to correlate with fever,
with higher amounts of virus shed when temperatures are highest.
Children are much more infectious than adults and shed virus from just
before they develop symptoms until two weeks after infection.
The transmission of influenza can be modeled mathematically,
which helps predict how the virus will spread in a population.
Influenza can be spread in three main ways: by direct
transmission (when an infected person sneezes mucus directly into the
eyes, nose or mouth of another person); the airborne route (when someone
inhales the aerosols produced by an infected person coughing, sneezing
or spitting) and through hand-to-eye, hand-to-nose, or hand-to-mouth
transmission, either from contaminated surfaces or from direct personal
contact such as a hand-shake.
The relative importance of these three modes of transmission
is unclear, and they may all contribute to the spread of the virus.
In the airborne route, the droplets that are small enough
for people to inhale are 0.5 to 5 µm in diameter and inhaling just one
droplet might be enough to cause an infection. most of these droplets
are quite large and will quickly settle out of the air.
Doorknobs, light switches and other household items.
However, if the virus is present in mucus, this can protect it for
longer periods (up to 17 days on banknotes). They are inactivated by
heating to 56 °C (133 °F) for a minimum of 60 minutes, as well as by
acids (at pH <2).
Knowing which genes are carried by a particular strain can
help predict how well it will infect humans and how severe this
infection will be (that is, predict the strain's pathophysiology).
For instance, part of the process that allows influenza
viruses to invade cells is the cleavage of the viral hemagglutinin
protein by any one of several human proteases.
Strains that are easily transmitted between people have
hemagglutinin proteins that bind to receptors in the upper part of the
respiratory tract, such as in the nose, throat and mouth. In contrast,
the highly-lethal H5N1 strain binds to receptors that are mostly found
deep in the lungs.
This difference in the site of infection may be part of the
reason why the H5N1 strain causes severe viral pneumonia in the lungs,
but is not easily transmitted by people coughing and sneezing.
Common symptoms of the flu such as fever, headaches, and
fatigue are the result of the huge amounts of proinflammatory cytokines
and chemokines (such as interferon or tumor necrosis factor) produced
from influenza-infected cells.
In contrast to the rhinovirus that causes the common cold,
influenza does cause tissue damage, so symptoms are not entirely due to
the inflammatory response. This massive immune response might produce a
life-threatening cytokine storm.
This effect has been proposed to be the cause of the unusual
lethality of both the H5N1 avian influenza, and the 1918 pandemic
strain. However, another possibility is that these large amounts of
cytokines are just a result of the massive levels of viral replication
produced by these strains, and the immune response does not itself
contribute to the disease.
It is possible to get vaccinated and still get influenza.
The vaccine is reformulated each season for a few specific flu strains
but cannot possibly include all the strains actively infecting people in
the world for that season.
It takes about six months for the manufacturers to formulate
and produce the millions of doses required to deal with the seasonal
epidemics; occasionally, a new or overlooked strain becomes prominent
during that time and infects people although they have been vaccinated
(as by the H3N2 Fujian flu in the 2003–2004 flu season).
It is also possible to get infected just before vaccination
and get sick with the very strain that the vaccine is supposed to
prevent, as the vaccine takes about two weeks to become effective.
Vaccines can cause the immune system to react as if the body
were actually being infected, and general infection symptoms (many cold
and flu symptoms are just general infection symptoms) can appear,
though these symptoms are usually not as severe or long-lasting as
The most dangerous side-effect is a severe allergic reaction
to either the virus material itself or residues from the hen eggs used
to grow the influenza; however, these reactions are extremely rare.
In addition to vaccination against seasonal influenza,
researchers are working to develop a vaccine against a possible
The rapid development, production, and distribution of
pandemic influenza vaccines could potentially save millions of lives
during an influenza pandemic.
Due to the short time frame between identification of a
pandemic strain and need for vaccination, researchers are looking at
non-egg-based options for vaccine production.
Live attenuated (egg-based or cell-based) technology and
recombinant technologies (proteins and virus-like particles) could
provide better "real-time" access and be produced more affordably,
thereby increasing access for people living in low- and moderate-income
countries, where an influenza pandemic may likely originate.
As of July 2009, more than 70 known clinical trials have
been completed or are ongoing for pandemic influenza vaccines.
In September 2009, the US Food and Drug Administration
approved four vaccines against the 2009 H1N1 influenza virus (the
current pandemic strain), and expect the initial vaccine lots to be
available within the following month.
Reasonably effective ways to reduce the transmission of
influenza include good personal health and hygiene habits such as: not
touching your eyes, nose or mouth; frequent hand washing (with soap and
water, or with alcohol-based hand rubs); covering coughs and sneezes;
avoiding close contact with sick people; and staying home yourself if
you are sick.
Avoiding spitting is also recommended.
Although face masks might help prevent transmission when
caring for the sick, there is mixed evidence on beneficial effects in
Smoking raises the risk of contracting influenza, as well as
producing more severe disease symptoms. Thus, according to the laws of
mathematical modelling of infectious diseases, smokers raise the
exponential growth rates of influenza epidemics and may indirectly be
responsible for a large percentage of influenza cases.
Since influenza spreads through both aerosols and contact
with contaminated surfaces, surface sanitizing may help prevent some
Alcohol is an effective sanitizer against influenza viruses,
while quaternary ammonium compounds can be used with alcohol so that
the sanitizing effect lasts for longer.
In hospitals, quaternary ammonium compounds and bleach are
used to sanitize rooms or equipment that have been occupied by patients
with influenza symptoms.
During past pandemics, closing schools, churches and
theaters slowed the spread of the virus but did not have a large effect
on the overall death rate.
It is uncertain if reducing public gatherings, by for
example closing schools and workplaces, will reduce transmission since
people with influenza may just be moved from one area to another; such
measures would also be difficult to enforce and might be unpopular.
Children and teenagers with flu symptoms (particularly
fever) should avoid taking aspirin during an influenza infection
(especially influenza type B), because doing so can lead to Reye's
syndrome, a rare but potentially fatal disease of the liver.
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