Pneumonia is a group of lung infections that can cause mild to severe disease, and is one of the leading causes of mortality worldwide. The main organisms that cause pneumonia include bacteria, viruses, and fungi. Depending on the organisms that cause the infection, the symptoms that patients experience can vary. Other factors that may affect the symptoms that patients suffer from include age and other health conditions. Patients with pneumonia normally present with coughing, fever, difficulty breathing, chest pain, and a loss of appetite.
Patients are advised to seek medical help immediately if their symptoms do not improve or if they worsen over time, especially shortness of breath, coughing, and fever. Low blood pressure and confusion are two other warning signs signifying that patients should visit the doctor immediately.
According to the World Health Organisation, pneumonia is a group of infections that can lead to acute inflammation in the lungs. From 2009 to 2019, an average of 2.52 million deaths occur each year due to pneumonia. Amongst these, about 77.5% are from children under 5 years old and adults above 70 years old. Understanding the causes of pneumonia, and being able to recognise the signs and symptoms of pneumonia early, allows prevention methods and treatment to be carried out promptly.
Causes of pneumonia
There are several organisms that commonly cause pneumonia, including viruses, bacteria, and fungi. These different infectious agents lead to different types of pneumonia infections, and the symptoms and treatment may vary accordingly. However, it is not always possible to diagnose the exact infectious agent causing the infection because of the varied symptoms that the patient may present with and the difficulty in extracting lower respiratory tract samples.1
Pneumonia is usually spread from one patient to another through droplets containing the infectious agent that patients expel, for example, during coughing. People may come into contact with these droplets in crowded spaces or in households with pneumonia patients. Some infectious agents can also be transmitted from soil or water to humans.
Pneumonia can also be caused by foreign substances, such as foods and fluids, entering the respiratory tract and down the lungs. This form of pneumonia is known as aspiration pneumonia and is more common in patients who have difficulties swallowing, are under anaesthesia, or have compromised immune systems. The microorganisms in the inhaled foreign substances, such as bacteria, may disturb the balance of microorganisms in the lungs and promote inflammation, causing lung infections.2
Some people may have higher risks of getting pneumonia compared to others because they have certain risk factors. Examples of these risk factors are:
- A compromised immune system, e.g. HIV patients.
- Age - children below 2 years old and adults of 65 years and above have higher risks because babies’ immune systems are not fully developed, whereas old age can weaken a person’s immune system.
- Chronic diseases, such as diabetes or heart diseases, and chronic respiratory diseases, such as chronic obstructive pulmonary disease (COPD)3 and asthma.
Types of pneumonia
Generally, pneumonia can be categorised into 4 types according to the infectious agents that caused the infection:
The most common type of bacterial pneumonia is pneumococcal pneumonia caused by the bacteria Streptococcus pneumoniae (or pneumococcus).4 Other bacteria that typically cause bacterial pneumonia include Staphylococcus aureus, Klebsiella pneumoniae, and Haemophilus influenzae. Research has also shown that it is possible to have infections caused by more than one type of bacteria at the same time. Doctors will usually prescribe antibiotics to treat bacterial pneumonia.
From human rhinoviruses (HRV) that cause the common cold to SARS-CoV-2 that caused the COVID-19 pandemic, viruses are also one of the main contributors to pneumonia. Influenza A is one of the main causes of community-acquired pneumonia (CAP). Some, but not all, viral pneumonia can be treated by antivirals. Certain viruses can help bacteria in transmission and infection, which makes treating and diagnosing pneumonia more challenging.5
Fungal pneumonia is more common in immunocompromised patients than in the general population because fungi generally only cause lung infections when the patient’s immune system is weak. Aspergillus species, Cryptococcus, and Pneumocystis jirovecii are some of the most common fungi that cause pneumonia. Pneumonia caused by P. jirovecii is categorised as an ‘AIDS-defining illness’, which acts as one of the diagnostic criteria for AIDS in HIV-positive patients.
Atypical pneumonia is so called because the imaging patterns of the patients’ respiratory system appear unusual and different from the patterns observed in ‘typical pneumonia’ caused by common pneumonia bacteria.6 Examples of atypical pathogens are Mycoplasma pneumoniae, Legionella pneumophila, Chlamydia psittaci, and Chlamydia pneumoniae. Atypical pneumonia is also sometimes known as ‘walking pneumonia’ because the symptoms are milder and patients can still carry out daily activities. Bacteria M. pneumoniae is usually the cause of ‘walking pneumonia’.
Pneumonia can also be classified according to the environment in where it is caught:
Community-acquired pneumonia (CAP)
CAP is pneumonia caught outside of hospital and healthcare settings. CAP is the leading cause of hospitalisations and mortality, and nearly 50% of CAP cases are caused by bacteria S. pneumoniae.7
Hospital-acquired pneumonia (HAP)
HAP is pneumonia acquired at least after 48 hours of being in a hospital setting. The main causes of HAP are microaspiration (inhaling of fluids from the stomach due to reflux) and contamination of hospital equipment. The main infectious agents that cause HAP are bacteria Pseudomonas aeruginosa and S. aureus.
Signs of pneumonia
If you have contracted pneumonia, you may show the following symptoms:
- A dry cough or a cough with thick mucus
- Dyspnea (shortness of breath)
- Pleuritic chest pain (sharp pain during inhalation due to friction between the tissue layers that line the lungs and the chest wall)
- Loss of appetite
Patients with mild symptoms generally get better after taking medication prescribed by a doctor, drinking plenty of water, and getting enough rest. Doctors may decide to admit a patient to the hospital if they think the condition is severe, based on information such as oxygen levels in the blood, x-ray images, white blood cell and platelet counts, and body temperature.8
Some symptoms are less common and may be more specific for certain types of pneumonia. For example, some fungal pneumonia can cause the patient to cough up blood along with other symptoms mentioned above, while atypical pneumonia can also cause muscle pains and headaches.9
The symptoms that a patient experiences differ between individuals due to variables such as age and personal health. Older patients may experience an altered state of consciousness4 or confusion, while this symptom is generally less common among younger patients.
When to seek medical attention
According to Cleveland Clinic, if your symptoms do not get better for 3-5 days or even worsen, or if you have developed new symptoms, you should visit the doctor immediately. If you experience worsening shortness of breath, coughing, fever, a drop in blood pressure, or you begin to experience confusion, you should seek medical attention right away.
People who are suspected to have pneumonia may be advised to get a chest X-ray. Doctors may also collect blood samples to measure the c-reactive protein level, which can help indicate the likelihood of pneumonia compared to other respiratory conditions.10 Pulse oximetry is a diagnostic test that may be used to test a patient's oxygen level in blood. Further tests such as polymerase chain reaction (PCR) tests may be required to identify the microorganism causing the infection.
Pneumonia is a group of infections of the lungs that are caused by various microorganisms. The symptoms caused by the different infectious agents may vary slightly, but generally, patients experience coughing, shortness of breath, chest pain, and fever. If symptoms become worse or show no improvements for up to five days since they first appeared, visit the doctor right away to receive immediate treatment.
- Mackenzie G. The definition and classification of pneumonia. Pneumonia [Internet]. 2016 Aug 22 [cited 2022 Aug 17];8(1):14. Available from: https://doi.org/10.1186/s41479-016-0012-z
- Mandell LA, Niederman MS. Aspiration pneumonia. New England Journal of Medicine [Internet]. 2019 Feb 14 [cited 2022 Aug 18];380(7):651–63. Available from: https://doi.org/10.1056/NEJMra1714562
- Kurashima K, Takaku Y, Nakamoto K, Takayanagi N, Kanauchi T, Yanagisawa T, et al. Risk factors for pneumonia and the effect of the pneumococcal vaccine in patients with chronic airflow obstruction. Chronic Obstructive Pulmonary Diseases:Journal of the COPD Foundation [Internet]. [cited 2022 Aug 19];3(3):610–9. Available from: http://doi.org/10.15326/jcopdf.3.3.2015.0167
- Eshwara VK, Mukhopadhyay C, Rello J. Community-acquired bacterial pneumonia in adults: An update. Indian J Med Res [Internet]. 2020 Apr [cited 2022 Aug 18];151(4):287–302. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7371062/
- Galván JM, Rajas O, Aspa J. Review of non-bacterial infections in respiratory medicine: viral pneumonia. Arch Bronconeumol [Internet]. 2015 Nov [cited 2022 Aug 19];51(11):590–7. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7105177/
- Dueck NP, Epstein S, Franquet T, Moore CC, Bueno J. Atypical pneumonia: definition, causes, and imaging features. RadioGraphics [Internet]. 2021 May [cited 2022 Aug 19];41(3):720–41. Available from: https://pubs.rsna.org/doi/full/10.1148/rg.2021200131
- Brown JS. Community-acquired pneumonia. Clin Med (Lond) [Internet]. 2012 Dec [cited 2022 Aug 19];12(6):538–43. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5922594/
- Jones BE, Jones JP, Vines CG, Dean NC. Validating hospital admission criteria for decision support in pneumonia. BMC Pulm Med [Internet]. 2014 Sep 22 [cited 2022 Aug 19];14:149. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4189199/
- Bedi R. Community acquired pneumonia - Typical or atypical ? Lung India [Internet]. 2006 [cited 2022 Aug 19];23(3):130. Available from: https://journals.lww.com/10.4103/0970-2113.44406
- Ruiz-González A, Utrillo L, Bielsa S, Falguera M, Porcel JM. The diagnostic value of serum c-reactive protein for identifying pneumonia in hospitalized patients with acute respiratory symptoms. Journal of Biomarkers [Internet]. 2016 Aug 16 [cited 2022 Aug 19];2016:e2198745. Available from: https://www.hindawi.com/journals/jbm/2016/2198745/