Pneumonia and Pulmonary Edema

Bacterial and Viral Pneumonia

Simon Duffy

Learning Objectives

By the end of this section, you will be able to:

  • Identify clinically relevant forms of bacterial and viral pneumonia
  • Compare and contrast major characteristics of different bacterial pneumonia
  • Compare and contrast major characteristics of different viral pneumonia

Pneumonia involves an acute respiratory infection that is most commonly caused by viruses or bacteria. The consequence of infection is inflammation, which results in an accumulation of fluid and immune cells in the alveoli. As the alveoli fill with fluid, breathing becomes painful and oxygen intake is limited.

Bacterial Pneumonia

Pneumonia involves infection of the lungs, which results in inflammation, accumulation of fluid and migration of immune cells into the alveolar space. Bacterial pneumonia is both prevalent and can potentially contribute to serious illness. A common cause of bacterial pneumonia is Streptococcus pneumoniae (pneumococcus). Children and elderly are most affected by this disease, and it is estimated that one million children die every year from pneumococcal disease.

Examples of Bacterial Pneumonia

Streptococcus pneumoniae (also known as Pneumococcus)

Most common cause of community-acquired bacterial pneumonia. In addition to pneumonia, pneumoncoccus accounts for half of cases of bacterial meningitis and can cause post-infection arthritis. Fortunately, the availability of pneumococcal conjugate vaccine (PCV13) is thought to prevent over 60,000 deaths every year[1].

Pneumococcal bacteria evade ingestion by macrophages and other phagocyte immune cells because the cell surface is encased in a polysaccharide capsule. The bacterium also releases pneumolysin O protein that forms pores in host cells, contributing to inflammation and bloody sputum. Some pneumococcal bacteria have developed resistance to penicillins and similar β-lactam antibiotics, requiring alternative antibiotics such as macrolides and fluoroquinalones.

Haemophilus influenzae type b (Hib)

Hib cause serious infection, especially in infants. In addition to pneumonia, meningitis is the most common clinical manifestation. Death occurs in 3-6% of cases and survivors may experience hearing impairment. The incidence of invasive Hib has decline by 99% since the introduction of routine childhood immunization.

Klebsiella pneumoniae

These bacteria may be found in the intestine and nose of healthy individuals and are often spread by contamination of hands. The rates of people who are carriers for this pathogen are particularly high in hospital environments. Immune compromised individuals may develop pneumonia, which is associated with 30-50% mortality. Part of the reason for this poor prognosis is due to increasing antibiotic resistence among these bacteria.

Pseudomonas aeruginosa

These bacteria are commonly found in soil, water and throughout the environment. P. aeruginosa can cause serious bacterial pneumonia in patients with cystic fibrosis because these patients have a genetic defect that impairs mucociliary clearance. P. aeruginosa also cause pneumonia in patients that rely on an artificial ventilator because the bacteria are aspirated directly into the lung.

Viral Pneumonia

Pneumonia caused by viral infection occurs more frequently than bacterial pneumonia and result in infectious diseases, such as the flu (influenza virus) and COVID-19 (SARS-CoV-2 virus). Some people may experience viral infections of the naval cavity, called the “common cold”. These cold viruses include rhinoviruses, adenoviruses and coronaviruses. Some cold viruses can disseminate from the nasal cavity to the lung to produce pneumonia.

Most viral pneumonias do not have a specific treatment but people typically get better on their own. While antibiotics are used to treat bacterial infection, these same medications are not effective against viruses. However, a viral pneumonia may place a person at greater risk of also contracting a bacterial infection, which would then have to be treated with antibiotics.

Examples of Viral Pneumonia


Adenoviruses can cause infection of the digestive or urinary tract but must frequently contribute to respiratory illness. Adenoviruses are relatively resistant to disinfection and can spread on surfaces (e.g. doorknob, light switch). Respiratory illness can range from  cold  to pneumonia, croup and bronchitis.

Influenza Virus

Influenza virus can infect the nose, throat or lungs. It causes a respiratory illness called flu, which typically has more abrupt and intense symptoms that than the common cold. Also, people with the flu are more likely to experience fever and chills than people with the common cold. Flu can progress to pneumonia and may contribute to death in people with a weakened immune system.

Flu can spread from person to person by droplets made when people cough, sneeze or speak. These droplets harbouring virus can infect people up to 2 meters (6 feet) away or can contaminate surfaces. Flu is a vaccine-preventable disease and immunization significantly reduces a persons likelihood of needing medical support or hospitalization, following infection.

Human Parainfluenza Virus (HPIV)

People may get re-infected with HPIV multiple times in their life but those most frequently infected include infants, young children and people with a weakened immune system. Infected people may experience a range of illness, from clod-like symptoms to croup, bronchitis, bronchiolitis and pneumonia. HPIV is spread by droplets that fall within 2 meters (6 feet) or may remain airborne for over an hour.

Respiratory Synyctial Virus (RSV)

One of the most common childhood illnesses, RSV infection causes a cold-like illness. In some cases, this infection may contribute to pneumonia. Severe infection in infants, young children and those who are immune compromised may require hospitalization.


Seven types of coronavirus are known to infect people. Four types cause mild-to-moderate upper respiratory infection. Three types (SARS-CoV, SARS-CoV-2, MERS-CoV) can cause severe pneumonia potentially resulting in death. SARS-CoV-2 is responsible for COVID-19, an infectious disease that spread as a global pandemic that contributed to the death of over 15 million people[2].

Review Questions

  1. Chapman, R., Sutton, K., Dillon-Murphy, D., Patel, S., Hilton, B., Farkouh, R., & Wasserman, M. (2020). Ten year public health impact of 13-valent pneumococcal conjugate vaccination in infants: a modelling analysis. Vaccine, 38(45), 7138-7145.
  2. Wang, H., Paulson, K. R., Pease, S. A., Watson, S., Comfort, H., Zheng, P., ... & Murray, C. J. (2022). Estimating excess mortality due to the COVID-19 pandemic: a systematic analysis of COVID-19-related mortality, 2020–21. The Lancet, 399(10334), 1513-1536.


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Pathology Copyright © 2022 by Simon Duffy is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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