Influenza, commonly known as the flu, is a respiratory illness caused by viruses. There are many different types of flu, from swine flu to the most feared avian flu. This article, however, deals only with the so-called common or seasonal flu. The flu season usually starts in the fall and subsides in late spring. Because of its seasonality, the flu season differs between the Northern and the Southern hemisphere.

On the global scale, flu can affect tens of millions of people and cause 250,000 to 500,000 deaths each year. The United States CDC gives the following statistics. Yearly 5 to 20% of the population get the flu, 200,000 cases are hospitalized due to severe flu complications and 36,000 cases prove to be fatal. Annual healthcare costs approaches US$10 billion

THE FLU is caused by types A, B and C. Each type consists of different subtypes and substrains with A and B being responsible for most epidemics.

(1) Influenza A viruses are of subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). The current subtypes of influenza A viruses in humans are A (H1N1) and A (H3N2).

(2) Influenza B viruses do not have subtypes but can be further broken down into different strains.

(3) Influenza C seems to be relatively benign compared to the first two and is not as contagious.

Like many viruses, influenza viruses change with time producing new strains each season. This change occurs by antigenic drift or shift.

(1) Antigenic drift is slow and can lead to new strains after several seasons and is the typical change observed in influenza type B viruses but can also occur in type A.

(2) Antigenic shift happens abruptly and creates a major change that can lead to a new subtype but only occurs in type A.

The influenza viruses attack the epithelial cells lining the respiratory tract starting from the nose, throat, and down to the lungs. They do this by binding using hemagglutinin and neuraminidase proteins on the cells' surface.

Influenza symptoms may be mild to severe to life-threatening. Symptoms can include high fever, headache, chills, sore muscles, fatigue, dry cough, red, watery eyes, sore throat and runny or stuffy nose. The so-called "tummy flu" symptoms may also occur in children, which include abdominal pain, vomiting, and diarrhea.

The flu symptoms shouldn't be confused with the ordinary cold, which is another respiratory illness. A cold usually doesn't cause high fever, muscle pains or stomach symptoms.

The incubation period of the flu can vary from a few days to a week. During this "asymptomatic period", the infected person is already a carrier of the disease and is therefore highly contagious. The flu can last from a few days to less than two weeks. However, in certain cases, complications can develop which are potentially life-threatening. Some of these complications are bronchitis, pneumonia, sinusitis, and ear infections. Anybody can get the flu but those who are highly susceptible to the flu are young children, the elderly and those with existing chronic health conditions and weakened immune systems.

Flu is very contagious. Direct transmission occurs from person to person via droplets that are released when a person coughs or sneezes. Indirect transmission happens when droplets from an infected person are transferred to inanimate objects, which are then touched by another person.

Flu-like symptoms can also occur with many other diseases. It is therefore sometimes difficult for doctors to diagnose the illness. There are many laboratory tests which can confirm whether a person has the flu or not but they cost money and time and are therefore impractical. However, in times of outbreaks and epidemics, it is important to perform tests. Usually, flu cases are diagnosed based on tests performed in a few individuals within a group. If one or two persons test positive for the flu in a closed environment, then the likelihood of subsequent illnesses within the group as being flu is rather high. Samples for flu testing are usually secretions from the nose and the throat, collected either by sterile swab, nasal wash, or aspiration. There are rapid influenza tests which can give results in 30 minutes. The tests are 70% sensitive in detecting the flu and are prone to false negatives. Another alternative is the more reliable but time consuming viral culture which takes about 3 to 10 days.

Several antiviral pharmacological agents have been approved for the treatment of the flu. Because of the seasonal changes in the types and strains of influenza viruses the recommended anti-flu drugs may change. This season, Tamiflu is the choice for treatment and chemoprophylaxis of persons aged 1 year and older and Relenza is approved for treatment of persons aged 7 years and older. Currently, the CDC discourages the use of previously used anti-flu drugs such as Amantadine and Rimantadine because the current circulating strains of flu viruses are resistant to these drugs.

The ultimate preventive measure against the flu is vaccination. However, again due to the constantly changing nature of the influenza viruses, flu vaccines do not afford 100% and lifetime immunity against the flu. Each season, flu vaccines are updated by adding strains collected and identified from the previous season. Flu vaccines contain different strains of the influenza types A and B but not type C.

Based on the 2005 guidelines of the Federal Advisory Committee on Immunization Practices (ACIP), annual flu shots are recommended for children aged 6 months to 5 years old and adolescents up to 18 years old. Vaccination is also highly recommended for people in key professions which may expose them to the infection and spread it to others. These include health care professionals and those working and living in closed units such as dormitories. Flu shots are usually given prior to flu season and afford protection for about a year. However, it takes about two weeks before the protection kicks in.

Over the years, several types of flu vaccines have been developed.

(1) The intranasal vaccine is the live, attenuated influenza vaccine (LAIV) made from living but weakened flu viruses. LAIV is prescribed for people aged 2 to 49 years old. LAIV should not be given to pregnant women, children under 2 years old, adults above 40 years old, and chronic medical conditions including compromised immune system disorders.

(2) The inactivated vaccine, the common flu shot is made from different strains of killed flu viruses. This vaccine is recommended for children from 6 months to 18 years old as well as those older than 50 years old. In addition, people who are not qualified to receive LAIV (as listed above) are recommended to have this vaccine.

Like all pharmacological agents, flu shots can have some side effects. Most of these side effects are mild. Rare severe side effects, mainly in the form of allergic reactions have been reported.

Some inactivated flu vaccines have contained thimerosal in the past, a form of mercury used as preservatives for vaccines. In recent years, there have been concerns about the link between thimerosal and autism in children. This controversy continues although no ample scientific evidence is available to prove this link.

Perhaps the main shortcoming of the flu shot is its efficacy, which is highly dependent on how well the vaccine is matched to the circulating influenza viruses of that season. For this reason, the efficacy of the vaccine in protecting healthy adults from the flu is only 70 to 90% and even less in children and the elderly.

The Global Influenza Surveillance Network (GISN) was set up in 1952 and currently includes more than 110 laboratories in 83 countries. It has recently set up FluNet, a web-based tool for surveillance, reporting, and alert mechanisms. The European Influenza Surveillance Scheme (EISS) issues a weekly report on influenza activity in 30 countries. It collects data from a network of more than 25,000 sentinel doctors covering a total population of 498 million people in Europe. In the US, the CDC is responsible for national influenza surveillance and is dependent on reports from more than 1500 sentinel doctors.

Google Flu Trends, being the new kid on the block, claims that it can estimate flu activity up "to two weeks faster than traditional flu surveillance systems" and bases its predictions on the number of people searching for flu-related topics. Actually its data compared well with the CDC's. Google is now working together with the CDC in flu surveillance and their work has been published in the prestigious scientific journal Nature. Google Flu Trends also presents the data in graphs, maps, and animations which are easy to comprehend by the general public.

Don't forget the CDC's initiative "Take 3" Steps To Fight The Flu; take time to get a flu vaccine, take everyday preventive actions and take flu antiviral drugs if your doctor recommends them.

And if you are still unlucky and get the flu, stay home from work or school including limiting contact with others, wash your hands often with soap and water and avoid touching your eyes, nose or mouth.