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Influenza, more commonly known as the flu, is a viral illness that affects the respiratory tract.

What is going on in the body? 
There are 3 types of influenza viruses: influenza A, B, and C. Each of these has several subtypes. All these forms of the virus mutate (change their genes slightly) and vary from year to year. This is why it is possible for the same person to have repeated episodes of the flu. Flu is usually transmitted from person to person through drops of respiratory discharge.

What are the signs and symptoms of the infection? 
Symptoms of the flu can include: What are the causes and risks of the infection? 
The flu strikes about 7 out of 1,000 people. It is highly contagious. Flu is much more common among people who live in institutional settings such as nursing homes. Those who come in contact with many other people in schools, day care centres, or large group organisations are also more likely to become infected.

What can be done to prevent the infection? 
An injection of flu vaccine every year can provide protection for people who want to avoid the flu. The flu vaccine is produced roughly 9 to 12 months ahead of the flu season. The formula used is based on what experts believe will be the most common strains of flu virus in the coming season. The vaccine has a 60% to 70% success rate in preventing the different types of influenza viruses each year.

The vaccine is recommended for people considered to be at high risk. These include:
  • people over age 65
  • children and adults with heart disease and lung disease, including asthma
  • people who live in nursing homes or other institutional settings
  • people who have a chronic disease such as diabetes, asthma, anaemia, or kidney disease
  • people who can transmit the flu to others at high risk. This group includes healthcare workers and employees at facilities caring for people at high risk.
  • women who will be in the second or third trimester of pregnancy during the flu season
  • people whose immune system is weakened because of chemotherapy
  • people with HIV or AIDS
A recent study showed that the flu vaccine might also be effective in preventing second heart attacks in people who have already had a heart attack. In the study, people who received the flu vaccine had a 67% lower incidence of a second heart attack than the people who were not vaccinated that year.

Elderly individuals who are vaccinated against the flu have a significantly lower incidence of hospitalisation for respiratory disease, congestive heart failure, and death from any cause.

About 25% of adults who receive the flu vaccine report mild soreness at the site of the injection. Young children may develop fever after a flu vaccine. There is also a slight risk of an allergic reaction to the vaccine. A nasal vaccine for the flu is being tested and may soon be available.

How is the infection diagnosed? 
A history and physical examination is the first step in diagnosing flu. Most of the time, the doctor can make the diagnosis without further testing. In some cases, laboratory tests may be ordered, including:
  • a culture of samples of discharge taken from the nose or back of the throat to determine the type of virus
  • the cold agglutinins test, which measures germ-fighting chemicals that cause blood to clump at low temperatures
  • the influenza complement fixation test, which can detect the presence of disease-fighting chemicals made by the body
What are the long-term effects of the infection? 
While most people recover from the flu without problems, others can develop complications such as:
  • pneumonia, an acute lung infection that can be caused by a virus or bacteria
  • acute bronchitis, an infection of the windpipe
  • secondary bacterial infections, which are infections caused by other germs besides those causing the flu infection
  • congestive heart failure
Influenza and its complications can cause serious disease or even death.

What are the risks to others? 
The influenza virus is highly contagious. It is transmitted through respiratory secretions. Sneezing and coughing can spread these droplets. The virus can also be passed on when an individual with the flu touches his or her nose and then handles an object that another person later touches. The second person can then pick up the virus from the object and transfer it into his or her own respiratory tract by touching the face.

What are the treatments for the infection? 
Flu is generally treated by addressing its symptoms. Bed rest and drinking plenty of fluids help prevent some complications. Antibiotics do not work against the viruses and can reduce the body's ability to fight viruses. Medications such as paracetamol, ibuprofen, or aspirin can help reduce fever and muscle aches. However, several recent studies showed that people who used fever-fighting medications such as aspirin and paracetamol had flu symptoms several days longer than those who did not use these medications.

More recently antiviral drugs such as Relenza have been released in Australia for the treatment of Influnza A and B viruses. Although effective it must be taken early. Likewise it has little effect on the common cold.

What are the side effects of the treatments? 
Side effects of fever-fighting medications such as ibuprofen, aspirin, and paracetamol include stomach upset and allergic reactions. Amantadine can cause anxiety, depression, and insomnia. Zanamivir or Relenza can cause airway spasm and reduced airflow in people with chronic respiratory disease.

What happens after treatment for the infection? 
Flu symptoms go away in 7 to 10 days for most people. Some people can develop complications, however. The flu can cause pneumonia, even in healthy people.

How is the infection monitored? 
A bout of flu usually clears up within 7 to 10 days. Any new or worsening symptoms should be reported to the doctor.

Author: James Broomfield, MD
Reviewer: HealthAnswers Australia Medical Review Panel
Editor: Dr David Taylor, Chief Medical Officer HealthAnswers Australia
Last Updated: 1/10/2001
Potential conflict of interest information for reviewers available on request

This website and article is not a substitute for independent professional advice. Nothing contained in this website is intended to be used as medical advice and it is not intended to be used to diagnose, treat, cure or prevent any disease, nor should it be used for therapeutic purposes or as a substitute for your own health professional's advice.  All Health and any associated parties do not accept any liability for any injury, loss or damage incurred by use of or reliance on the information.

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