Many community-acquired pathogens are associated with pneumonia, but Streptococcus pneumoniae is the most common bacterial cause. Mycoplasma pneumoniae is associated with outbreaks every 4 years or so in the UK and is much more common in school-aged children.⁸

A pathogen historically associated with pneumonia is Haemophilus influenzae, but the H influenzae type b (Hib) vaccination has seen pneumonia due to this decline. Staphylococcus aureus can cause complicated pneumonia with potentially worse outcomes than with S. pneumoniae.⁹

Viral causes, usually influenza or respiratory syncytial virus (RSV) and most recently Covid-19, account for around 13% of pneumonia cases in adults, but potentially 66% of cases in children, particularly RSV. Viral infections can also increase the risk of worsening bacterial pneumonia.^2,3,9

Prior to the introduction of the RSV vaccine programme in September 2024, RSV accounted for around 30,000 hospitalisations in children aged under 5 and was responsible for 20 to 30 infant deaths each year. It also caused around 9,000 hospital admissions annually in those aged over 75.¹⁰

Early onset HAP, starting within 4 days of admission, usually involves the same bacteria and viruses seen in CAP and has a good prognosis. Late onset HAP, starting 5 or more days after admission is more likely to originate from a microorganism picked up in hospital and has a worse prognosis.³