What causes pneumonia?

Drawing of lungs in chest with lungs highlighted

Pneumonia is an inflammation of the lung tissue (shown in orange) caused by an infection with bacteria or a virus. Image: Sebastian Kaulitzki / Shutterstock.com

Pneumonia in adults

Pneumonia in adults is usually caused by bacteria.

The flu (influenza) virus and the respiratory syncytial virus (that can cause bronchiolitis) can also cause pneumonia.

Fungi can also cause pneumonia, but this is very rare in people who are generally healthy. Fungal infections that can cause pneumonia are histoplasmosis, coccidioidomycosis, blastomycosis and pneumocystis pneumonia (a lung infection mainly occurring in people with weakened immune systems such as in those who are HIV positive).

Pneumonia in children

Babies aged 1 week old or less: bacterial infections are the most common cause of pneumonia in this age group. The source of the infection is usually the bacteria found in and around the mother’s birth canal.

Babies aged 1 week to 4 months: most cases of pneumonia at this age are caused by a virus. Bacteria can also cause pneumonia, which can be life threatening in babies this young. Chlamydia trachomatis (C. trachomatis) is a type of bacteria that can cause pneumonia in babies up to 3 months old. Half of all children who have C. trachomatis pneumonia will also have conjunctivitis (an eye infection also known as ‘pink eye’).

Children aged 4 months to 5 years: most cases of pneumonia in this group are caused by a virus. Respiratory syncytial virus (RSV), and sometimes the flu (influenza) virus are common viral causes of pneumonia.

Children aged 5–15 years: bacterial infections are the most common causes of pneumonia in this age group, with Streptococcus pneumoniae the most common cause. Mycoplasma pneumoniae also causes pneumonia in school-aged children, as well as in younger children.

The bacterium Staphylococcus aureus can cause severe pneumonia in children of all age groups, but this is not common.

Knowing the likely cause of a child’s pneumonia helps doctors decide on the best treatment.

Find out more about the diagnosis and treatments for pneumonia in adults and children.

References
  • Antibiotic Expert Group. Community-acquired pneumonia in adults. In: eTG complete [online]. Therapeutic guidelines: antibiotic. Melbourne: Therapeutic Guidelines Limited; 2012. (Accessed 27 March 2012).
  • Antibiotic Expert Group. Community-acquired pneumonia in children. In: eTG complete [online]. Therapeutic guidelines: antibiotic. Melbourne: Therapeutic Guidelines Limited; 2012. (Accessed 27 March 2012).