What is Influenza (Flu)?

InfluenzaInfluenza is an acute respiratory illness caused by the influenza virus which exists as three types: A, B or C.
Influenza type A is found in humans as well as other mammals and birds. It is the only type that has been known to cause influenza pandemics (widespread outbreak of disease).
Influenza type B occurs in humans and seals.
Influenza type C occurs in humans and pigs. Influenza type C occurs commonly in children under 6 years of age, and causes a mild illness. Most humans are exposed to influenza type C as children and therefore develop immunity to it.
Tiny proteins are present on the envelope that surrounds the influenza virus particles. These are known as the hemagglutinin (H) and the neuraminidase (N). Several types of H and N exist, and the combination of these changes over time. For example bird flu is H5N1, while swine flu is H1N1. Influenza vaccines are designed to prevent viruses with particular H and N combinations, which is why a vaccine that works one year will not be effective for preventing flu the following year.
Influenza is a highly contagious infection, with outbreaks of the illness being commonplace, particularly during winter months. Large numbers of virus particles are present in respiratory tract secretions (such as sputum/phlegm or saliva), resulting in transmission of small aerosolised particles that spread though sneezing, coughing and talking, and are subsequently inhaled by the next person to become infected.
Alternatively, contact transmission may take place, when infected individuals touch mucous membranes (soft skin in the mouth or nose) or respiratory secretions (cough or sneeze into their hand), before direct or indirect (touching common surfaces) interpersonal contact with a new host who deposits the virus on their own mucous membranes.
Uncomplicated influenza infection occurs when virus particles attach to the upper respiratory tract (windpipe and upper airways). If the virus particles extend further down into the lungs, they cause the most common complication of influenza; pneumonia. The lungs may then become infected by bacteria, and this is known as secondary bacterial pneumonia. Other rarer complications may occur, such as spread of the virus to cause disease in the heart, muscles or brain.


During influenza outbreaks, 10-20% of the general population may become infected. During pandemics, up to 50% of the population may become infected. Seasonal influenza causes 200,000 hospitalisations and 41,000 deaths in the US every year. It is the seventh leading cause of death in the US.
InAustralia, there were 44,221 laboratory confirmed cases of the flu in 2009. Each year an average of 2,686 deaths occur due to influenza infection.
Around 20-43% of all children are infected during typical influenza seasons, with the highest incidence amongst children less than 2 years old.13 Between 1997 and 2005, 41 children died inAustralia due to influenza infection.
Deaths due to influenza are not common, but the rates are highest in elderly patients, followed by infants.
Influenza is the cause of 5 to 12% of pneumonia that occurs in the community.

Risk Factors

Like other respiratory viruses, influenza is spread through tiny particles of respiratory secretions in the air therefore exposure is more likely in overcrowded or poorly ventilated environments.
The influenza vaccine is advised for the following list of people, due to their increased risk of exposure to the virus, or their increased risk of serious complications if exposure does occur. In Australia, immunisation is recommended for:

  • All adults aged over 65 years;
  • Infants and children aged from 6 months to 4 years;
  • Children on long term aspirin therapy;
  • Pregnant women (especially those who are in their second or third trimester between June and October);
  • Anyone with a chronic medical condition (e.g. asthma, diabetes or heart, kidney or lung disease);
  • Anyone with a weakened immune system or who is undergoing immunosuppressive treatments ;
  • Residents of nursing homes or long term care facilities;
  • Child care workers, health care workers, or anyone living with or looking after someone at high risk of developing flu-related complications; and
  • Anyone visiting parts of the world where influenza is circulating.


InfluenzaWhen an individual is exposed to the influenza virus, viral particles enter the lining of their respiratory tract and begin to multiply. The newly formed viral particles then begin ‘shedding‘ off the lining and entering secretions such as phlegm and saliva. Healthy adults began shedding the flu virus one half to one day after they were exposed, with a sharp increase to peak shedding on the second day, followed by a rapid decline.Average duration of shedding is 4.8 days, though it continues for 10 days in some individuals. Children, the elderly, those with chronic illnesses and immuno-compromised hosts may shed for longer.While someone is shedding the virus, they are able to spread it to other people.
The incubation period (during which a person is infected with the virus but doesn’t experience any symptoms) is 1 to 3 days. Symptoms then start suddenly, as discussed below.
In the case of uncomplicated influenza, these symptoms generally improve over two to five days, though may last one or more weeks. Some patients experience postinfluenzal asthenia (persistent weakness or becoming tired easily) which may be present for several weeks following the illness. A dry cough (post viral cough syndrome) may also linger for several weeks.
In particularly nasty (virulent) influenza strains, or in susceptible hosts, complicated influenza may occur. The major complication of influenza is pneumonia.


Pneumonia occurring as a complication of influenza infection may be due to the virus itself (primary viral pneumonia) or a bacterial infection following the influenza (secondary bacterial pneumonia). Primary viral pneumonia is the least common and the most severe of these categories. Secondary bacterial pneumonia, though less severe, contributes to approximately 25% of all influenza-associated deaths.
Influenza related pneumonia occurs most commonly in the following high risk groups:

  • Those with respiratory or cardiovascular co-morbidities;
  • Those who are immunosuppressed, or have a diagnosis of diabetes  mellitus, renal (kidney) disease or hemoglobinopathy;
  • Individuals aged over 50; and
  • Individuals who reside in care facilities such as nursing homes.

Myositis and rhabdomyloysis

Muscle pain is commonly associated with influenza, however true myositis, in which the virus damages muscle tissue is a rare complication, and tends to occur in children.

Other complications

Many other less well documented complications of influenza infection have been reported. Such complications include  encephalitis affecting the brain, Guillain-Barre syndrome, aseptic meningitis, transverse myelitis, toxic shock syndrome, and myocarditis or pericarditis affecting the heart.


Young children may have a wide range of symptoms that resemble other conditions such as bronchiolitis, croup, or otitis media (middle ear infection). Rarely febrile convulsions (fitting associated with high temperature) may occur.Viral particles have been known to travel to muscles resulting in myositis.
Reye’s syndrome is a rare complication that occurs principally in children with viral illness who are administered aspirin. Presenting features of Reye’s syndrome are vomiting and confusion progressing to coma. Brain injury or death may ensue. Aspirin should therefore not be used in children who have influenza.


Influenza is different from the common cold in that it causes a more severe illness, with fever, headache, significant fatigue and muscle aches. It is less likely to cause sneezing or a ‘blocked nose’ with thick nasal discharge.
Following an incubation period of 1-2 days, flu presents with abrupt onset of fever, muscle aches, headache and fatigue. The individual may have respiratory symptoms such as a dry cough, sore throat, and occasionally a runny nose. Other symptoms related to systemic illness include chills and sweats, loss of appetite and diarrhoea and vomiting,which is more common in children.
There are a wide variety of presentations depending on the strain of influenza and the characteristics of the person who is infected. These include mild respiratory illness without fever, or significant systemic symptoms without respiratory symptoms. These symptoms generally improve over two to five days, though may last one or more weeks. Some patients experience postinfluenzal asthenia (persistent weakness or easy fatigability) which may be present for several weeks following the illness.
Patients who develop pneumonia may become gradually worse, or experience initial improvement in the symptoms of acute influenza, followed by worsening of fever and respiratory symptoms, with development of a moist cough. Pneumonia may also present as a high fever with shortness of breath and a dry cough.
Patients who develop acute myositis experience extreme muscle tenderness, usually in the legs. If severe, swelling and bogginess of affected muscle groups may occur.

Clinical Examination

InfluenzaThe doctor may take your temperature, and look in your throat and nose. They may also listen to your lungs with a stethoscope to check for pneumonia. Individuals who are infected with influenza usually feel ‘unwell’ and have a hot, flushed appearance. The redness in the throat may be mild, even if it is very sore. Lymph nodes in the neck may be mildly swollen. The nose may be runny, but discharge is usually absent. Eyes may be red and watery, and a dry cough may be present. Heart rate may be increased due to fever.

How is it Diagnosed

In otherwise healthy individuals who develop influenza during outbreaks, no tests are generally required to diagnose the illness. If there is thought to be a pandemic risk, testing may occur for public health purposes. If a person develops a severe respiratory infection, and influenza is one of the possible causes, testing may be carried out to make sure the correct treatment is given. Testing usually involves the doctor taking swabs from the back of the nose and back of the throat, which are then sent to the laboratory to identify whether influenza is present, and if so, what type.
If the doctor suspects you have developed pneumonia, further tests such as blood tests, sputum specimens, and chest x-rays may be required.


InfluenzaAlthough people who have influenza feel very unwell while they are affected, the infection usually goes away on its own, and does not result in any long term problems. In this instance, the illness is termed ‘uncomplicated influenza’. However, in vulnerable hosts, or those infected with a particularly nasty strain, complicated influenza may occur, resulting in significant illness and in rare cases, death.
Hospitalisation and deaths are known to occur mainly within high risk groups (as outlined previously). Annual epidemics result in between 3 to 5 million cases of severe illness, and around 250,000 to 500,000 deaths every year. In non pandemic flu, most deaths occur in adults aged over 65. Not much is known regarding the prognosis of influenza in non industrialised countries.
A recent study examined the outcomes for children in South Australia who required hospital admission due to influenza between 1996 and 2006. 649 children required hospital admission during the 11 years. Children aged less than two years made up 72% of these admissions, and children under 6 months made up 24.8%. The average length of hospital stay was 2.5 days. 45 children required ICU admission, with 20 children requiring mechanical ventilation. Four children died from influenza related deaths during the study period. Three of these were known to have other medical conditions, and were aged younger than 8 months.


Treatment of uncomplicated influenza in otherwise healthy individuals includes:

  • Resting in bed until the fever subsides;
  • Use of paracetamol for pain control; and
  • Drinking plenty of fluids

Antibiotics are not useful in treating uncomplicated influenza in healthy individuals. Antibiotics are designed to kill bacteria, so they are not helpful in treating illnesses caused by viruses, such as influenza. Antibiotics should only be prescribed if complications occur, such as bacterial pneumonia.
Oseltamivir, also known as Tamiflu (capsule or suspension) and zanamivir, also known as Relenza(inhaled powder) may be used in selected patients to treat influenza A and influenza B. Treatment with oseltamivir results in shortening of symptom duration by one day, and reduces time to return to work by half a day.The main role of these drugs is to reduce the likelihood of complications, such as pneumonia, associated with influenza infection. They must be taken within 48 hours of onset of symptoms to be effective. The earlier treatment starts, the less severe and shorter the illness. Adverse effects include potential bronchospasm (wheeze) associated with inhaled zanamivir and headache and nausea and vomiting associated with oseltamivir.
Oseltamivir may also be used for influenza prevention during outbreaks if it is given within two days of exposure to the virus.
Amantadine and rimantadine are both antiviral drugs that have been used to treat influenza in the past, however their use is now limited as the virus may not be susceptible to them.
The influenza vaccination is an important way to prevent infection and spread of the illness. It is not effective once someone has already caught the infection. Because the common type of influenza varies each year, each vaccine is only effective for 12 months. The vaccine is 50-80% effective, though this varies from year to year. It is designed to prevent the most common varieties of influenza circulating that year, and is most effective for preventing these strains, though it still has some affect in reducing infection with different strains.


The influenza vaccine is advised for the following list of people, due to their increased risk of exposure to the virus, or their increased risk of serious complications if exposure does occur. In Australia, immunisation is recommended for:

  • All adults aged over 65 years;
  • Infants and children aged from 6 months to 4 years;
  • Children on long term aspirin therapy;
  • Pregnant women (especially those who are in their second or third trimester between June and October);
  • Anyone with a chronic medical condition (e.g. asthma, diabetes or heart, kidney or lung disease);
  • Anyone with a weakened immune system or who is undergoing immunosuppressive treatments;
  • Residents of nursing homes or long term care facilities;
  • Child care workers, health care workers, or anyone living with or looking after someone at high risk of developing flu-related complications; and
  • Anyone visiting parts of the world where influenza is circulating.

Healthcare workers are a group who are at increased risk of contracting influenza, with up to 25% contracting the illness during flu season. This is important because not only do these workers become sick, and unable to carry out their jobs, but they may also unintentionally expose their patients to the illness. Influenza vaccination of healthcare workers is probably the best way to prevent this occurring. For this reason, specialist groups in Australia are working hard to ensure as many healthcare workers as possible receive the flu vaccine.

Influenza vaccination

For more information on how to prevent influenza infection, see Tips to Prevent Colds and Flus and Influenza Vaccination.

More information


For more information on the common cold and influenza, types of influenza and treatments and tips for preventing influenza, see Cold and Flu.


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