Flu or influenza is an infectious disease caused by the flu virus. Although the disease is not severe in most cases, it can lead to dangerous complications for certain risk groups. The vaccine is the best way to prevent complications from flu and admission to hospital.

What is flu? 

Flu is an infectious disease that is caused by the flu virus (influenza virus).
The disease is characterized by the development of flu syndrome. This is a combination of symptoms that are often associated with the disease, namely:

  • fever
  • cold chills
  • malaise
  • dry cough
  • loss of appetite
  • muscle pain
  • nausea.

The flu virus spreads more quickly among children due to poor hand and cough hygiene as well as the high rate of contact among children. Moreover, children are normally the first to be affected by seasonal flu, with adults only developing it a few weeks later.
Although the outcome of flu is usually favorable, complications such as lung inflammation, acute respiratory distress syndrome (ARDS) and deterioration of chronic conditions are possible in risk groups.

Whenever a virus is new to the population, other groups can be affected. 

We distinguish between several types: 

  • A-viruses, which can cause a pandemic (human and animal viruses)
  • B-viruses, which only occur in humans
  • C-viruses, which only emerge sporadically and usually cause mild symptoms.

The vaccine against seasonal flu only comprises A and B viruses.


In general, the doctor will not have to conduct a diagnostic test. The doctor makes a diagnosis based on the symptoms shown by the patient.

The doctor can still opt for a diagnostic test for people who are at risk or people for whom antiviral therapy is being considered. The doctor take this test from a nose and throat swab (respiratory sample) within 7 days of the start of the symptoms. He/she sends the sample to the laboratory to identify the virus type.

A blood test for antibodies is not suitable as a diagnostic test and also provides no guarantee of protection against a subsequent infection on account of the flu virus mutating regularly. A serological diagnosis is useful for checking the effectiveness of the vaccine and for epidemiological research (scientific research of diseases in a population according to various criteria: geographical and in terms of gender, method of development and transmission).


Recovery from flu is usually spontaneous.

However, some medicines may help to relieve symptoms such as muscle pain and headache.

Patients who are in a risk group and severely ill patients can be treated with antivirals. These medicines block the protein that allows the virus to multiply within the body. Caution is advised when prescribing antivirals. Otherwise the viruses could become resistant to them.

Antibiotics are not effective for the treatment of flu unless there are other complications. Antibiotics only work against bacteria. Flu is caused by a virus.

Risk Groups

People who are at risk have more chance of becoming severely ill and developing complications from a flu infection. 

People at risk: 

  • people aged 65 and older 
  • pregnant women 
  • people with chronic conditions (lung disorders, heart diseases, asthma, diabetes, low immunity, chronic kidney and liver diseases, neuromuscular conditions, obesity)
  • children who have a chronic illness or are receiving long-term treatment with aspirin.

Complications and death

For people in at-risk groups, flu can cause severe complications.

The most frequent complications are:

  • lung inflammation
  • ear infection
  • sinusitis.

Severe, more rare complications include:

  • myocarditis (inflammation of the heart muscle)
  • encephalitis (brain infection)
  • inflammation of muscle tissue
  • acute respiratory distress syndrome (a severe inflammatory response in the lungs)
  • septicemia (blood infection)
  • multiple organ failure.

Some of these complications are caused by an additional bacterial infection (secondary infection). Only in that case is treatment with antibiotics useful. The responsible use of antibiotics is required to safeguard their effectiveness in situations when they are really needed, such as with bacterial infections.

Death from flu is usually the result of one or more complications. During a seasonal flu epidemic, around 97% of the people who die from flu are aged 65 or over and 86% of the deaths from flu are people with an existing chronic health condition.

Prevention and vaccination 

Flu viruses are spread via the airways and survive on the hands for a long period of time. 

Prevention consists of avoiding contact with infected people. In addition, hygiene is very important: 

  • wash your hands regularly, especially after sneezing, coughing, coming into contact with an infected person or if you have traveled on public transport
  • sneeze into disposable paper tissues or into the crook of your elbow
  • use disposable paper tissues when you cough, spit out phlegm or blow your nose.

The vaccine is the best way to prevent the complications of flu and hospitalization. The effectiveness of the vaccine can vary considerably between seasons, however. More precisely, the effectiveness depends on the match between the vaccine and the circulating viruses (which develop mutations constantly) and on the person’s immune system.

What is a flu epidemic or flu pandemic? 

An epidemic means that a disease occurs with a greater frequency than normal in any given population.

We talk about there being a flu epidemic when a number of conditions have been met:

  • the number of doctor’s appointments due to a flu syndrome is higher than the epidemic threshold (in Belgium, this threshold is at round 140 appointments per 100,000 inhabitants per week)
  • the epidemic threshold is exceeded for at least 2 weeks in a row
  • virological research finds that at least 20% of the respiratory samples analyzed test positive for flu viruses.

A seasonal flu epidemic lasts 6 to 12 weeks on average.  In the northern hemisphere, the flu virus causes epidemics in the winter, usually between December and April. In the southern hemisphere, the flu virus causes epidemics between May and September. 

Seasonal epidemics can be caused by influenza-A and influenza-B viruses, but only the influenza-A virus causes pandemics.

A (flu) pandemic is an epidemic that spreads across several continents as a result of the emergence of a new virus.

In the case of flu, this is usually due to a new variant of the flu virus that the population has not yet come into contact with, meaning they have not yet developed any antibodies. As a result, there is a high risk of it spreading.

There have been various pandemics in the last century, each time a new flu virus emerged. 

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Information for health workers

Seasonal flu is not a notifiable disease

Sciensano publishes a weekly flu bulletin in Dutch or in French containing the latest Belgian figures.  

After the end of the season, an annual report is compiled containing information about the epidemiological status for the previous season and the effectiveness of the vaccine. 

Sciensano coordinates a sentinel network of general practitioners, laboratories and hospitals to ensure the permanent surveillance of influenza activity, of the intensity and severity of epidemics and the impact on the population. Sciensano is also the National Reference Centre (NRC) for influenza virus. The Belgian Official Medicines Control Laboratory (OMCL) of Sciensano, together with the European OMCL network, is responsible for the quality control of the influenza vaccine prior to marketing.

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