Acute respiratory tract infection

Acute respiratory infections occur in the form of seasonal peaks and can be caused by a large number of different germs. They cause a high burden of disease and have a severe social impact during epidemics.

What are acute respiratory infections?

Acute respiratory infections can be caused by many different germs (e.g. influenza virus , Respiratory Syncytial Virus (RSV), Mycoplasma pneumoniae, parainfluenza virus, adenovirus, human metapneumovirus, etc.). The disease is mainly transmitted by inhaling contaminated airborne droplets or particles.


The symptoms often include coughing, fever, shortness of breath, runny nose, sore throat and generally feeling unwell. It is not possible to know which pathogen is responsible on the basis of the symptoms alone.

It is relatively common for complications to appear such as pneumonia or, more rarely, acute respiratory distress syndrome (ARDS). Especially in certain risk groups (the elderly, people with a chronic condition and infants), these acute respiratory infections can lead to a serious burden of disease and even death. The higher overall mortality during the winter months is partly due to acute respiratory infections.

Common pathogens

Infections by certain germs often occur in the form of an annual epidemic. However, the moment of the peak of infections depends on the germ (see table).

Some common causes of acute respiratory infections are:

Pathogen Incubation period Most affected age group Symptoms and complications Annual peak
Influenza virus 1-5 days Children < 15 years
  • fever
  • cough
  • shortness of breath
  • general feeling of being unwell
  • pneumonia
  • acute respiratory failure
January — March
RSV 2-8 days Children < 2 years
  • fever
  • cough
  • sneezing
  • wheezing
November — January
Para-influenza virus 2–6 days Children < 5 years
  • runny nose
  • cough
  • fever


In severe cases:

  • croup
  • lower respiratory tract infection (bronchitis)
  • pneumonia

Peak in:

  • March-June (serotype 3)
  • October-December (serotype 1)
Adenovirus 2–14 days (5-6 days for acute respiratory infection)  
  • common cold
  • sore throat (pharyngitis)
  • fever
  • eye infection (conjunctivitis)
  • ear infection
  • lower respiratory tract infection (bronchitis)
  • pneumonia
  • bladder infection
  • digestive complaints


In people with a weakened immune system or young children, the infection can be serious:

  • pneumonia
  • hepatitis
  •  myocarditis
  • meningitis
  • encephalitis
Present all year round, but peak in November-March
Mycoplasma pneumoniae 1-4 weeks School-age children and young adults
  • mild progression
  • cough
  • general malaise
  • skin rash
  • fever
  • sore throat (pharyngitis)
  • lower respiratory tract infection (bronchitis)


In some cases, the progression of the disease can be severe:

  • pneumonia
  • encephalitis
  • meningitis
  • myocarditis
  • pericarditis
  • anaemia
  • arthritis
Epidemic progression peaking every 4-7 years, usually in the winter months

Sciensano coordinates a network of sentinel doctors, sentinel laboratories and sentinel hospitals in order to evaluate continuously the intensity and severity of acute respiratory infections and their impact on the population. Sciensano is also the National Reference Centre (NRC) for the influenza virus. Sciensano’s Belgian Official Medicine Control Laboratory (OMCL), together with the European OMCL network, is responsible for the quality control of the influenza vaccine before it is marketed.

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