Last Review Date: November 6, 2017

What is influenza?

Influenza (the flu) is a viral respiratory infection that spreads from person to person through coughing, sneezing, and contact with contaminated surfaces. The flu is caused by types A, B, or rarely C influenza virus. The most common cause is influenza A, the viral culprit behind flu pandemics and most epidemics. In colder climates, influenza is seasonal, occurring primarily in the winter. In warmer regions of the world, it may be present year-round. During each flu season, there are multiple strains of influenza present, but typically one or two strains predominate as that year's "seasonal flu."

Influenza A and B viruses change over time. Seasonal influenza strains can undergo a series of genetic changes so that people no longer have immunity from prior infections or vaccination. When a large number people are susceptible to the virus, it can cause an influenza epidemic. In addition to this, influenza A can undergo a major genetic change that can make a virus strain much more lethal and/or easier to transmit. Flu vaccines that are developed each year to prevent flu infection are based upon experts' opinions, working in conjunction with the World Health Organization, as to which strains are likely to circulate in the community and usually contain attenuated or inactivated virus targeting two influenza A strains and one influenza B strain.

The influenza virus causes illness in humans and in many animals, including birds, pigs (swine), dogs, and horses. Human influenza strains pass easily from person to person, but most strains of animal influenza only rarely infect humans. When they do, it is almost exclusively when there is significant close animal contact, such as a person that raises chickens or pigs, and the subsequent infection is only rarely transmitted from the infected person to another person.

The ongoing worry for the world's medical communities is that an influenza strain that is infecting animals such as birds or pigs will mutate sufficiently that it will cause serious illness and death in humans (who have no protective antibodies against it) and that it will become a strain that is transmitted easily from human to human.

Influenza A virus can be further sub-typed based on two unique protein antigens, H (haemagglutinin) and N (neuraminidase). The most common influenza A viruses currently infecting humans have the subtypes H1N1 and H3N2. Additional descriptors are used to identify a particular influenza virus in more detail, this is a typically a combination of place where the virus was first observed, strain number, and year. For instance, the predominant strain during the 2003-2004 flu season was influenza A/Fujian/411/2002 (H3N2).

In most cases, the specific name of the influenza virus is only relevant to the medical community and those charged with influenza surveillance, but in recent years there has been news and focus on first avian (bird) flu and then H1N1 (swine) flu.

Avian (Bird) Flu
The avian flu, an influenza A, H5N1, virus was first described in Hong Kong in 1997. It caused an epidemic in birds in Southeast Asia in 2004, and since then it has caused illness and deaths in birds and in some people in parts of Asia. So far this strain of influenza has remained a bird-to-human infection.

H1N1 (Swine) Flu
The 2009 H1N1 flu virus was originally called "swine flu" but it is now known to be a combination of human, swine, and avian flu genes. First reported in Mexico and the U.S., it is a new influenza A, H1N1, virus. It became the  predominant influenza A virus worldwide in 2009, causing influenza infections throughout the world, in what is known as a “pandemic”.  

With the usual seasonal flu, the highest infection rates are seen in the very young, but more than 90% of influenza-related deaths and 60% of hospitalizations occur in those over 65 years old. In cases reported to European Centre for Disease Prevention and Control during the 2009 H1N1 pandemic, however, nearly 80% of those with severe respiratory infection from pandemic influenza who died were people under 65 years. The 2009 H1N1 flu virus is the latest, but not the most severe pandemic in history. For comparison, the most lethal pandemic in recent history, the 1918-1919 H1N1 influenza A pandemic, infected about one third of the world's population (an estimated 500 million people) and killed an estimated 20 to 50 million persons worldwide.

According to the Australian influenza report in 2011 there were 25,092 confirmed (laboratory test positive) cases of influenza reported to the National Notifiable Diseases Surveillance System (as at 16 October 2011). Estimating the actual numbers of flu cases is difficult, however, because many of those who get the flu do not seek medical treatment and, of those who do, only a small number are tested. Testing is more common in people who are hospitalized, but overall, laboratory-confirmed cases of influenza only represent a small percentage of those in a community who actually have the flu. In 2012, H3N2 Influenza infections began to increase again in the Northern Hemisphere, displacing the predominant H1N1 strain, illustrating the continual variation in global Influenza virus patterns over time. These patterns will continue to be monitored in Australia and worldwide.

Signs and Symptoms

For most people, the seasonal flu is a moderate illness that causes symptoms such as fatigue, fever, chills, stuffy nose, sore throat, headache, muscles aches, weakness, a cough and, with some strains, even diarrhoea and vomiting. These symptoms may also be seen with a variety of other conditions and seasonal viral infections.

Influenza can be severe and lead to complications such as viral pneumonia or secondary bacterial pneumonia (see Pneumonia) in the very young, the elderly, in those who are pregnant, and in those with underlying conditions such as asthmalung diseaseheart diseasediabeteskidney disease, liver disease, chronic neurological disease, and in those with compromised immune systems.

The incubation period for influenza is about two days. This is followed by several days of illness and then a resolution of symptoms. People shed virus and are infectious about a day before symptoms emerge and then for about five to seven days, or until about 24 hours after their fever ends.

Children and those with compromised immune systems may be infectious for longer periods of time.

What is the difference between influenza and the common cold?
While some bad colds can cause most of the symptoms above except diarrhoea and vomiting, in general, cold symptoms are milder than influenza. People with influenza generally have a moderate to high fever and muscle aches and pains. They feel very sick and need to stay in bed. Many people with a common cold are able to “soldier on” and continue daily activities even though they feel unwell. There are more than 100 viruses that can cause symptoms of the common cold, mostly rhinoviruses and coronaviruses. Because of this huge number of potential causes it is sometimes more difficult to identify the virus responsible for any particular common cold than it is to diagnose influenza.


Influenza tests are performed to diagnose the flu, to distinguish it from other viral infections or other causes of symptoms, to guide treatment, and to monitor the spread of influenza through communities and throughout the world.

Many individuals with the flu are not tested. They either do not seek medical treatment or their doctor makes the diagnosis of probable-flu based upon the person's symptoms, the fact that it is the flu season, and based upon the presence of influenza in the community. This is partially because rapid influenza tests will not detect every case of influenza and partially because more sensitive tests sometimes take too long to be useful to guide treatment. If it is to be useful in helping doctors make decisions about treatment, testing must generally be done within 48 hours of the onset of symptoms.

When rapid test results are negative, doctors may order follow-up testing but they will not wait for the results to initiate treatment. Testing is primarily performed on those who are seriously ill (hospitalized) and on those who are at risk for a severe case of influenza or for complications.

Laboratory Tests

Several different types of influenza tests may be used to detect an infection. These include:

Tests that can be done from a nasopharyngeal (nose and throat) swab or aspirate:

  • Rapid influenza antigen test – used to detect influenza A or influenza A and B and to guide antiviral treatment. Can be performed in a doctor's office with results often available in 30 minutes. This test is very quick but not as sensitive as the methods below.
  • Direct fluorescent antibody stain (DFA) – more sensitive than rapid testing but requires specialized training to interpret.
  • Viral culture – considered the gold standard for diagnosing influenza but can take 3 to 7 days. Can detect influenza A and B and determine the strain of influenza. This is now rarely done in laboratories.
  • Real-time polymerase chain reaction (RT-PCR) – a sensitive test that detects viral genetic material. A specific version of this test has been developed to detect the 2009 H1N1 Influenza virus, as well as tests designed to detect other Influenza A and Influenza B virus strains.

Blood tests:

  • Influenza A or B antibody test – blood tests performed to detect the body's immune response to an influenza infection. These tests are positive after recent influenza infection and are typically done for research or public health purposes.

For more on these, see the article Influenza Tests.

Non-Laboratory Tests
  • Chest X-ray - sometimes done to evaluate the lungs of a person with symptoms of pneumonia.


The best means of handling influenza is to prevent getting the infection in the first place, which is accomplished through seasonal influenza vaccination and through actions taken to minimize its spread. In Australia, annual influenza vaccination is strongly recommended in the following groups who are at an increased risk of influenza complications:

  • Persons aged 65 years and older
  • Persons aged 6 months and older with medical conditions that can lead to complications from influenza
  • Pregnant women
  • Persons aged 6 months to less than 5 years, or 15 years and older who identify as Aboriginal or Torres Strait Islander

These persons can access Influenza vaccination funded by the Immunise Australia Program. For full details, visit the Immunise Australia website.

Actions that can be taken to minimize the spread of influenza include hand washing, cleaning potentially contaminated surfaces, coughing and sneezing into tissues, and, when ill, staying home and limiting contact with other people.

Many people who do get influenza have a moderate, self-limited illness and do not require medical treatment. Fluids, bed rest, and over-the-counter pain and fever reducing medications are used to relieve symptoms until the infection resolves.

Antibiotics are not effective against influenza but effective antiviral medications are available. If given, they should be started as soon as possible after the emergence of symptoms and can decrease the duration of symptoms and shedding of the virus when administered within the first 48 hours of the start of symptoms. These medications can lessen the severity and duration of the infection. The Australian government recommends treatment for those with confirmed or suspected influenza who have severe illness or who are at risk of severe complications. Those at high risk may be sometimes be treated before they become ill (antiviral chemoprophylaxis) if they have been in close contact with some who has influenza.

Those who develop secondary complications, such as bacterial pneumonia, will also require treatment with antibiotics.

Related Pages

On this site
Tests: Influenza Tests
Conditions: AsthmaLung DiseasesPneumoniaKidney DiseasesHeart DiseaseCystic Fibrosis,DiabetesPregnancy

Elsewhere on the web
NSW Health Influenza Factsheet 
Department of Health - Australian Influenza Surveillance Reports
Immunise Australia Program
Kid's Health Info (the Flu)

NPS MedicineWise - Influenza

World Health Organization: Influenza