- Pneumonia in newborn infants is usually due to a bacterial infection such as Group B Streptococcus and Gram negative bacilli (e.g. Klebsiella).
- Viruses especially the respiratory syncytial virus, cause most pneumonias in infancy.
- In young children Mycoplasma is a common cause of pneumonia.
- Pneumonia in older children is usually due to bacteria such as Pneumococcus, Haemophilus and Staphylococcus. Pneumococcus is the most common cause of community-acquired pneumonia in children.
- Tuberculosis is an important cause of pneumonia in poor communities.
- Pneumocystis is an important cause of pneumonia in HIV infected infants between 2 and 6 months of age. This is a very unusual cause of pneumonia in children who do not have AIDS.
- Gram negative organisms such as Klebsiella and E. coli are also an important cause of severe pneumonia in children with HIV infection.
Chlamydia can cause pneumonia in infants.
It is difficult to decide whether the pneumonia is due to a virus, bacteria or TB on both clinical examination and chest X-ray. Often pneumonia is due to bacteria complicating a viral infection.