Atypical pneumonia: causes, symptoms, diagnosis and treatment

Pneumonia is inflammation of the lungs, usually caused by infection. The NHS says it is usually caused by a bacterial or viral infection. The word atypical is a historical label used in clinical practice for pneumonia caused by organisms that differ from the common bacteria which typically cause a sudden, severe chest infection, and that often respond to different antibiotics. The label is not used as a formal diagnosis on the NHS patient pages cited here, which describe each infection by name, so this guide uses the sources' wording for each organism.

The most commonly discussed causes in the UK are Mycoplasma pneumoniae, Legionella (Legionnaires' disease) and Chlamydia psittaci (psittacosis). Others exist, and a clinician will decide which is likely based on symptoms, exposure history, tests and the person's overall health.

UKHSA describes Mycoplasma pneumoniae as a bacterium that causes acute respiratory illness, ranging from mild illness to severe pneumonia, which on rare occasions has been associated with severe complications such as encephalitis. It is found in people of all ages, though most laboratory reports are in children aged 5 to 14 and adults aged 30 to 39.

The NHS says Legionnaires' disease is a lung infection caught by inhaling droplets of water containing the bacteria, usually in places like hotels, hospitals or offices where bacteria have got into the water supply. Sources include air conditioning systems, humidifiers, spa pools and hot tubs, and taps and showers that are not used often. It is uncommon but can be very serious.

UKHSA says psittacosis is a bacterial infection caused by Chlamydia psittaci, which usually infects birds. It can be passed to humans mainly by breathing in infected respiratory particles or dust contaminated with droppings or secretions, and it is commonly associated with pet birds.

The NHS says pneumonia symptoms can start suddenly or gradually over a few days. They include a cough, shortness of breath, a high temperature, chest pain, an aching body, feeling very tired, loss of appetite and wheezing; confusion is common in older people.

For Legionnaires' disease the NHS lists a cough, shortness of breath, chest pain or discomfort, particularly when breathing or coughing, a high temperature and flu-like symptoms. UKHSA says psittacosis typically presents with non-specific flu-like symptoms such as fever, headache, muscle aches and cough. A gradual, flu-like start with a dry cough that drags on is a common story, which is partly why these infections are sometimes described informally as walking pneumonia, though people can also become seriously unwell.

The NHS advises an urgent GP appointment or NHS 111 if you have had a cough for up to 3 weeks, you are coughing up yellow or green mucus, you have chest pain that comes and goes or happens when breathing or coughing, or you feel short of breath. For Legionnaires' disease it also asks you to tell the doctor where you have been in the last 10 days, such as a hotel, spa or hospital.

It says to call 999 or go to A&E if you are struggling to breathe, choking or gasping and unable to speak, have pale, blue or blotchy skin, lips or tongue, are coughing up blood, suddenly feel confused, or if you cannot wake your baby or they feel floppy.

A GP can diagnose pneumonia from your symptoms and examination, with a chest X-ray and blood tests usually arranged if you need hospital care, according to the NHS. Identifying the organism needs laboratory testing. UKHSA explains that psittacosis is confirmed by PCR testing of respiratory samples, available from its reference unit, and that clinicians should consider it where there is exposure to birds or in severe undiagnosed pneumonia. UKHSA also runs reference and diagnostic services for Mycoplasma pneumoniae.

Telling your doctor about travel, hotel stays, hot tubs, bird contact and workplace exposure matters, because that history helps point towards the organism.

The NHS says pneumonia caused by bacteria is usually treated with antibiotics, and most people get better in 2 to 4 weeks. You should keep taking antibiotics until finished and contact your doctor if you do not feel better after 2 to 3 days. Treatment in hospital may involve antibiotics, sometimes steroids and oxygen. The NHS says you may need hospital treatment if you are over 65, have cardiovascular disease or a long-term lung condition, are very unwell or are not getting better on antibiotics.

For Legionnaires' disease the NHS says treatment in hospital may include antibiotics through a vein, oxygen and, sometimes, breathing support, followed by antibiotic tablets at home for 1 to 3 weeks in total, and that most people make a full recovery though it may take a few weeks to feel back to normal. UKHSA says psittacosis cases can be treated with appropriate antibiotics.

For self-care, the NHS suggests resting, drinking plenty of fluids, taking paracetamol or ibuprofen if suitable, and not smoking. It advises against cough medicine, because coughing helps your body get rid of the infection.

The NHS lists vaccines that help protect against infections which can cause pneumonia, including pneumococcal, flu, RSV and COVID-19 vaccines for those in eligible groups, and says stopping smoking also reduces your chance of getting pneumonia. These do not prevent every atypical cause. Keeping building water systems free of bacteria reduces the risk of Legionnaires' disease, and reducing the build-up of bird droppings and dust reduces the risk of psittacosis, as UKHSA describes.

Suspected pneumonia is assessed by GPs, NHS 111, urgent treatment centres and emergency departments on the NHS, with testing and hospital care free at the point of use. Private GP services and walk-in clinics can also examine you, arrange X-rays and prescribe antibiotics, but anyone who is breathless, confused or very unwell should be seen through urgent NHS care rather than waiting for a private appointment.