Also Known As
Avian flu
Swine flu
Influenza A
Influenza B
This article was last reviewed on
This article waslast modified on 19 February 2019.
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, which is why annual flu vaccination is recommended for persons at increased risk of severe flu-related symptoms. 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 medical experts' opinions 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.

Influenza viruses cause 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. Once this happens, a new worldwide flu pandemic could spread across the world.

Influenza A virus can be further sub-typed based on two unique protein antigens, H (hemagglutinin) and N (neuraminidase). The most common influenza A viruses currently infecting humans have the subtypes H1N1 and H3N2. New flu virus strains are named according to the 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
One of the most predominant recent avian flu strains, an influenza A, H5N1virus 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. In 2013, avian influenza H7N9 was first reported to have infected humans in 2013 in China. Reported cases of human infection have resulted in severe respiratory illness.

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 is currently the predominant influenza A virus and is causing influenza infections throughout the world.

So far, the 2009 H1N1 flu appears to have had a greater effect on those younger than 65. Between April and December 2009, about 90% of influenza-related hospitalisations and 88% of estimated deaths were in people younger than 65. However, if infected, very young children, pregnant women and the elderly are at increased risk of severe or fatal flu.

Estimating the actual numbers of flu cases is difficult 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 hospitalised, but overall, laboratory-confirmed cases of influenza only represent a small percentage of those in a community who actually have the flu.

For comparison with the 2009-2010 flu season, 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, with more than 500,000 deaths in the U.S. alone.


Accordion Title
About Influenza
  • 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 asthma, lung disease, heart disease, diabetes, kidney disease, liver disease, and in those with compromised immune systems as with cancer or HIV/AIDS.

    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.


  • Tests

    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 take too long to be useful to guide treatment. If it is to be useful in helping doctors make decisions about treatment, testing must be done within 48 hours of the onset of symptoms.

    When rapid test results are negative, doctors may request follow-up testing but they will not wait for the results to initiate treatment. Testing is primarily performed on those who are seriously ill (hospitalised) 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:

    • 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. This is the test most commonly used in the UK with testing done in hospital laboratories.
    • Rapid influenza antigen test – used to detect influenza A or influenza A and B and to guide antiviral treatment.. These tests are not currently widely used in the UK.
    • Direct fluorescent antibody stain (DFA) – more sensitive than rapid testing but requires specialised training to interpret. This test is only performed in hospitals on young children in the UK.
    • 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 test, although useful for epidemiology and vaccine production is used rarely in the UK.
    • Influenza A or B antibody test – blood tests performed to detect the body's immune response to an influenza infection. These tests 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.


  • Treatments

    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 minimise its spread. In the UK, the Departments of Health provide guidance for annual flu vaccination, with those people at greater risk of severe symptoms strongly recommended to be vaccinated each Autumn. Actions that can be taken to minimise 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.

    Antiviral medications are available to treat those who have influenza. 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 and will be recommended by GPs or hospital doctors according to the current Departments of Health Flu Guidelines. Those who develop secondary complications, such as bacterial pneumonia, will also require antimicrobial treatment.