What causes pneumonia? 

Pneumonia is not caused by a single germ but can be triggered by a wide variety of infectious organisms including bacteria, viruses, and, less commonly, fungi. The condition develops when these germs are inhaled into the lungs and overwhelm the body’s natural immune defences, leading to inflammation and fluid buildup in the air sacs. Identifying the specific cause is a vital step for clinicians, as the treatment for bacterial pneumonia is entirely different from that for viral pneumonia. 

What We Will Cover in This Article 

• The primary bacterial causes of lung infections 

• Common respiratory viruses that lead to pneumonia 

• Rarer causes, including fungal and aspiration pneumonia 

• How germs enter the lungs and cause inflammation 

• Risk factors that make the lungs more susceptible to infection 

• Emergency guidance for severe respiratory distress 

Bacterial causes of pneumonia 

Bacterial pneumonia is one of the most common forms of the condition and is often more severe than viral types. The most frequent cause in the UK is a bacterium called Streptococcus pneumoniae. Bacteria can spread through airborne droplets when an infected person coughs or sneezes, or they can travel to the lungs from the upper respiratory tract during a period of illness. 

Clinical observation shows that bacterial pneumonia often affects a specific area, or lobe, of the lung  a condition known as lobar pneumonia. While antibiotics are highly effective against these organisms, bacterial pneumonia requires prompt treatment to prevent the infection from spreading to the bloodstream. 

Bacterial Pathogen	Common Name/Source	Clinical Impact
Streptococcus pneumoniae	Pneumococcus	Most common cause of community  acquired pneumonia
Haemophilus influenzae	H. flu	Often affects those with existing lung conditions like COPD
Staphylococcus aureus	Staph	Can cause severe infection, often following a viral flu
Mycoplasma pneumoniae	Walking Pneumonia	Usually causes milder symptoms in younger adults

Viral and fungal causes 

Viruses are a frequent cause of pneumonia, particularly in children and the elderly. Often, a viral infection like the flu or the common cold starts in the upper respiratory tract and then moves down into the lungs. While viruses themselves cause inflammation, they also weaken the immune system, which can allow a secondary bacterial infection to take hold. 

Fungal pneumonia is much rarer and typically affects individuals with significantly weakened immune systems, such as those undergoing chemotherapy or living with HIV. These fungi are often found in soil or bird droppings and are inhaled into the lungs. 

• Influenza: The flu virus is a leading cause of viral pneumonia during winter months. 

• Respiratory Syncytial Virus (RSV): A common cause of pneumonia in infants and young children. 

• COVID  19: A coronavirus that can cause extensive inflammation throughout both lungs. 

• Pneumocystis jirovecii: A specific type of fungus that causes pneumonia in immunocompromised patients. 

Aspiration and non  infectious causes 

Pneumonia can also occur without a direct ‘catchable’ germ. Aspiration pneumonia happens when food, drink, vomit, or saliva is accidentally inhaled into the lungs instead of being swallowed into the oesophagus. This material carries bacteria from the mouth into the lungs, where it triggers an infection. This is more common in individuals who have difficulty swallowing or those with a reduced level of consciousness. 

Hospital  acquired pneumonia is another specific category. This occurs in patients who are already in the hospital for another reason. The germs found in hospital settings are often different from those in the community and may be more resistant to standard antibiotics, requiring specialized clinical management. 

To Summarise 

The causes of pneumonia are diverse, ranging from common bacteria and viruses to rare fungi and accidental aspiration. Understanding the cause is essential because it determines whether a patient needs antibiotics, antivirals, or supportive care. Regardless of the cause, the resulting inflammation of the air sacs is a serious health event that requires medical monitoring to ensure the lungs recover fully. 

If you experience a high fever with shivering, severe chest pain, or a cough that produces blood or dark green mucus, call 999 immediately. 

Authority Snapshot 

The clinical information regarding the causes of pneumonia, including the differentiation between bacterial, viral, and fungal types, is based on the NICE guidelines and the British Thoracic Society standards. These protocols ensure that healthcare providers identify the correct pathogen to provide targeted treatment. This article has been written to provide an accurate and safe overview of how lung infections begin and is reviewed by medical professionals for clinical accuracy. 

Dr. Stefan Petrov is a UK  trained physician with an MBBS and postgraduate certifications including Basic Life Support ‘BLS’, Advanced Cardiac Life Support ‘ACLS’, and the UK Medical Licensing Assessment ‘PLAB 1 & 2’. He has hands  on experience in general medicine, surgery, anaesthesia, ophthalmology, and emergency care. Dr. Petrov has worked in both hospital wards and intensive care units, performing diagnostic and therapeutic procedures, and has contributed to medical education by creating patient  focused health content and teaching clinical skills to junior doctors.