Acute lower respiratory infections

Acute lower respiratory infections include pneumonia (infection of the lung alveoli), as well as infections affecting the airways such as acute bronchitis and bronchiolitis, influenza and whooping cough. They are a leading cause of illness and death in children and adults across the world. The importance of lower respiratory infections may be underestimated.

Last Update 17/05/2023
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Acute bronchitis and bronchiolitis

Acute bronchitis is a short-term infection of the airways. Bronchiolitis is an infection of the lower airways affecting babies and infants under 2 years of age. It is the most common cause of admission to hospital for babies under the age of 1 year.


Influenza occurs in annual epidemics and occasionally pandemics, where the outbreak covers an even larger geographical area. The most serious forms of this infection are seen in people over the age of 65 years, children under the age of 2 years and people of any age with certain existing medical conditions. There are several underlying conditions that could increase the risk of hospitalisation from influenza. These include diabetes and heart, lung and neurological diseases, including asthma.


Pneumonia is an infection of the lung, the most frequent cause of death due to infection in Europe and the USA. It causes symptoms for 3-4 weeks and is more common in very young children and elderly adults. There are three types of pneumonia:

  • Community-acquired pneumonia (CAP), which is contracted from coming into contact with the infection in daily life.
  • Hospital-acquired pneumonia, which is contracted after a period of time in hospital.
  • Ventilator-associated pneumonia (VAP), which is acquired after a procedure called endotracheal intubation, when a tube is inserted into the trachea to help a person breathe.


Acute bronchitis and bronchiolitis

The main symptom of acute bronchitis is a cough that cannot be explained by an existing long-term lung condition. Bronchiolitis causes babies and infants to have prolonged symptoms of wheeze and cough, lasting for weeks and months after the first symptoms.


The usual form of influenza can result in symptoms of fever, muscle pain, headache, cough, sore throat and a blocked nose. If the infection is more severe it can lead to pneumonia.


People with pneumonia experience a cough along with other symptoms such as fever or breathlessness without an obvious cause.


Infections are caused by tiny organisms, known as bacteria or viruses. There are millions of organisms that surround us. Viruses are carried in tiny droplets and are passed between people when someone with the virus coughs or sneezes. People who are infected produce antibodies to fight the virus. Once they have fought off this strain, they will be able to fight it if infected again. However, over time, the virus can change into different forms, which means the body may not have the right antibodies to fight it again.

Acute bronchitis and bronchiolitis

Almost 90% of cases of acute bronchitis are related to viruses such as influenza and the rhinovirus. Less than 10% are linked with bacteria. Bronchiolitis is most often caused by respiratory syncytial virus (RSV) in infants and babies.


There are three influenza viruses A, B and C. A has the biggest impact, B causes less severe illness and C does not cause any illness in humans. When a new strain develops that people have no resistance to, it can spread across the world. This was the case during the ‘swine flu’ outbreak in 2009.


The most common cause of pneumonia in adults is a bacterium called Streptococcus pneumonia. Viruses can also cause pneumonia, most commonly the respiratory syncytial virus (RSV) in small children, and sometimes influenza. Hospital-acquired pneumonia is caused by germs in a hospital, whereas community-acquired pneumonia is caused by different germs in the community.


The best way to prevent infections is via vaccinations. Vaccination against pneumonia and influenza is commonly given to groups, such as the elderly or very young, who are most at risk.


One way to treat in an infection is through drugs known as antibiotics. Antibiotics should only be prescribed when necessary or antibiotic resistance can develop.

Antibiotic resistance is when bacteria is regularly exposed to the same drug over time and will eventually mutate and develop resistance to this drug. Using antibiotics to treat mild illnesses unnecessarily speeds up the problem of antibiotic resistance, than if the drugs were only used in severe cases.

  • Acute bronchitis and bronchiolitis

    People who suspect they have bronchitis do not need to visit a doctor for a diagnosis, unless they have severe or persistent symptoms. Most cases of acute bronchitis do not require drugs to treat the condition and the symptoms can be managed at home by getting rest and drinking fluids. Antibiotics have no effect on viruses so are not prescribed to treat acute bronchitis.

    Bronchiolitis is diagnosed by monitoring the symptoms and analysing a child’s breathing. Babies infected with the respiratory syncytial virus (RSV) usually require supportive care but no medication. In extremely severe cases, they are given antiviral drugs.

  • Influenza

    People who are fit and healthy do not need to visit a doctor to diagnose flu. It is usually possible to treat the symptoms of influenza at home with rest and drinking plenty of water. Antiviral drugs are also used to treat influenza in people who are at risk of other complications. A doctor will diagnose flu based on symptoms and medical history if a person is at risk.

  • Pneumonia

    An x-ray can be used to check for pneumonia and is likely to show shadowing on the lungs if the infection is present. Antibiotics are used to treat pneumonia and different approaches should be used according to the risk of the person and severity of the illness.