Introduction 

Pneumonia is a medical term used to describe infection within one or both of the lungs that can typically cause inflammation and fluid to build up within small balloon-shaped air sacs known as alveoli that are found at the bottom of the lungs.¹ It can be caused by infection with bacteria, fungi, or viruses. There are approximately 100 million cases of viral pneumonia per year and with the increase of viral pandemics like COVID-19, this figure is set to increase in the future. Therefore it is important to understand the most common causes of viral pneumonia.^2,3

What is viral pneumonia?

This lung infection not only produces fluid and inflammation but is also responsible for the abnormal exchange of carbon dioxide and oxygen within the alveoli, leading to a typical clinical presentation.⁴ General symptoms of pneumonia begin with a cough and congestion which can be heard when a doctor auscultates the chest, and this can be seen with or without a fever. Other common symptoms include headaches, fatigue, and abnormal lung sounds like wheezing, rales, and rhonchi.⁵

In terms of diagnosing viral pneumonia, bacterial pneumonia is first excluded as a cause which is usually treated using antibiotics. Treatment for viral pneumonia involves supportive management. There is an increased risk of viral pneumonia in certain populations, including the very young and elderly, as well as those who are pregnant or have weakened immune systems. 

The most common causes of viral pneumonia, as detailed in this article, include Influenza, Respiratory Syncytial Virus, SARS-CoV-2 (COVID-19), Adenovirus, and Parainfluenza virus.⁴

Common viruses that cause viral pneumonia

Influenza virus

Influenza, also commonly known as the ‘flu’, is a viral infection that can spread easily between people. It typically affects the throat, lungs, and nose. Influenza can be separated into three different categories: A, B, and C, depending on the RNA virus's physical structure. Influenza A tends to cause the most severe symptoms. Although influenza occurs throughout the year, there are more cases and pandemics in the winter months.⁶

How does it cause pneumonia?

Influenza can cause pneumonia due to the inflammation it causes in the lungs. This inflammation manifests into coughing, problems with breathing, and fever. It also increases mucus production and slows the airway’s ability to move air which can lead to a buildup of this mucus within the lungs.⁷

Symptoms

Symptoms of the influenza virus include:

• Persistent cough and sough throat 
• Feeling tired 
• Muscle pains 
• Headache 

These symptoms typically last for more than five days. As this worsens, symptoms include:

• Chest pain 
• Respiratory distress/dyspnoea 
• Cyanosis⁵

Who is most at risk?

The groups most at risk of influenza virus include:

• Older people
• Young children 
• Pregnant assigned female at birth (AFAB) people 
• People with chronic illnesses⁸

Treatment 

This is a complicated form of influenza so antiviral drugs like oral oseltamivir, Amantadine hydrochloride and rimantadine hydrochloride can be used straightaway.⁹

Respiratory syncytial virus (RSV)

Respiratory syncytial virus (RSV) is a respiratory infection that typically causes cold-like symptoms, and often due to its similarities, it can be mistaken for the flu, a cold, or even COVID. However, its most severe presentation affects infants and elderly populations, who in addition to pneumonia patients can develop bronchiolitis and breathing difficulties, leading to hospitalisation. It occurs mainly in the winter and is spread through aerosol droplets. The incubation period is the time delay before symptoms appear is around three to five days.¹⁰

How does it cause pneumonia?

When RSV is transmitted in its droplet form to a healthy person, it spreads usually to the nasopharyngeal mucosa and then throughout the respiratory tract. Cells like the apical ciliated epithelial cells are the targets and the virus binds to cell receptors and then replicates within the host cells. This produces a cascade of inflammatory immune responses and mucus production which obstructs airways and impairs cilia which usually clears the mucus. The mucus production plays the causative role in lower respiratory infections like bronchiolitis and pneumonia.¹¹

Symptoms

Symptoms of RSV include:

• Wheezing 
• Rhonchi 
• Non-productive coughs 
• Ear pain 
• Fever 
• Anorexia (loss of appetite)
• Dyspnoea⁵

Who is most at risk?

Groups most at risk from RSV include:

• Infants and young children
• Premature babies 
• Children with cystic fibrosis weakened immune systems, or neuromuscular disorders that make it difficult for them to swallow. 
• Adults over the age of 60 
• Adults with heart failure, chronic obstructive pulmonary disease and asthma
• Adults living in nursing homes 
• People with severe obesity 
• People with chronic diseases¹²

Treatment 

Treatment for RSV can include:

• Symptom support: oxygen, IV fluids, intubation 
• RSV immunisation for at-risk groups 
• Palivizumab, an antibody therapy
• Ribavirin for severe illnesses¹⁰

SARS-CoV-2 (COVID-19)

What is it?

Coronavirus causes severe acute respiratory syndrome and is a virus which has appeared in humans since 2004. The most recent Coronavirus disease 2019 was due to a new strain of the virus SARS-CoV-2. Like many other respiratory illnesses, it spreads by inhaling virus-containing droplets from an infected person and the individual is considered infectious 48 hours before the symptoms start. In most people, COVID can cause mild to moderate respiratory symptoms but in some individuals, it can progress and lead to hospitalisation.²

How does it cause pneumonia?

In most COVID-19 patients, symptoms are confined to the upper respiratory system, but in some patients, the type 2 epithelial cells in the lungs may become damaged and die when infected by the virus. These cells' normal function is to produce surfactant; without this surfactant, the lungs and alveoli within the lungs are at risk of collapsing which can lead to pneumonia.¹³

Symptoms

• Fever
• Dry Cough 
• Muscle pain 
• Dyspnoea 
• Confusion 
• Pleuritic pain 
• Gastrointestinal symptoms like diarrhoea

Other symptoms of COVID-19 which can coexist with these include loss of taste and smell.¹⁴

Who is most at risk?

Groups most at risk of COVID-19/SARS-CoV2 include:

• People over the age of 65
• Pregnant AFAB people 
• People with a weakened immune system 
• Smokers/people who have previously smoked⁴

Treatment

Treatment for COVID-19/SARS-CoV2 can include:

• Oral antiviral Paxlovid (nirmatrelvir/ritonavir) 
• Supportive treatment 
• Vaccination for prevention⁵

Adenovirus

What is it?

Adenoviruses are viruses responsible for causing upper and lower respiratory infections. In general, there are about 50 types of adenoviruses which are responsible for human disease. The incubation time before symptoms is between two days and two weeks. This virus also can remain latent inside the body and become active at a later stage. Typical transmission is from person to person either through close contact or inhaling aerosol droplets.¹⁵

How does it cause pneumonia?

Like other respiratory infections, adenovirus can infect the lung lining leading to inflammation and progressing to pneumonia or bronchiolitis.²

Symptoms

Adenovirus symptoms can include:

• Coughing 
• Wheezing 
• Sore throat 
• Bronchitis 
• Trouble breathing¹⁵

Who is most at risk?

Groups most at risk from adenovirus include:

• Transplant patients 
• Children under the age of 5
• Chemotherapy patients¹⁵

Treatment

Treatment for adenovirus may include:

• Fluids 
• Drugs to dilate the bronchi - open the airway
• Extra oxygen 

However, there are no FDA-approved antiviral drugs for pneumonia caused by adenovirus.¹⁶

Parainfluenza virus

Parainfluenza viruses are a group of RNA viruses that usually cause respiratory infections in those with weak immune systems. The incubation period is 2 to 6 days and the different types of parainfluenza virus are associated with different symptoms. For example, types 1 and 2 usually cause croup, type 3 causes pneumonia, bronchitis and bronchiolitis, and type 4 is associated with more severe forms of respiratory tract diseases. Parainfluenza is usually spread through airborne transmission and people can be reinfected multiple times in their life.¹⁷

How does it cause pneumonia?

Infections usually begin in the upper airways and replication of the virus occurs in the lower distal airways. Pneumonia is due to an increase in inflammatory infiltrations and overexpression of the immune system response.¹⁸

Symptoms

Symptoms of parainfluenza can include:

• Fever 
• Cough 
• Rhinitis 
• Wheezing 
• Chills⁵

Who is most at risk?

Groups most at risk from parainfluenza include:

• Infants younger than 6 months 
• Weakened immune systems¹⁷

Treatment

Treatment for parainfluenza may include:

• Support (additional oxygen, fluids, or intubation)
• Ribavirin is shown to be useful in immunocompromised patients⁵

Summary 

This article defines pneumonia as an infection within one or both of the lungs that can typically cause inflammation and fluid to build up within small balloon-shaped air sacs known as alveoli that are found at the bottom of the lungs and its association with viruses such as influenza, Respiratory Syncytial Virus (RSV), SARS-CoV-2 (COVID-19), adenovirus and parainfluenza virus.¹ 

These viruses typically cause lower respiratory tract infections and similar symptoms such as fever, dry cough, muscle pain, and dyspnoea, and can present with different sounds on auscultation such as wheezing, rhonchi, or rales. There are certain at-risk populations for these viruses which can include the elderly, young children, and the immunocompromised. Treatment for viruses in general is usually supported such as additional oxygen, fluids or intubation, but in some cases antiviral drugs can be useful.²