The Role of Bacterial Pneumonia in the Severity of the 1918 Influenza Pandemic

Randy Quill

The 1918 influenza pandemic stands as one of the most devastating outbreaks in human history. With millions of lives lost, understanding the factors that contributed to its severity is crucial for future pandemic preparedness.

Recent research suggests that bacterial pneumonia played a significant role in the mortality associated with the pandemic, overshadowing the primary viral infection.

In this blog, we explore the historical evidence and scientific insights that shed light on the impact of secondary bacterial pneumonia during the 1918 influenza pandemic.

Unveiling the Evidence:

Histological and bacteriological examinations of autopsy specimens from the 1918 pandemic revealed changes consistent with extensive bacterial pneumonia.

Culture results from international autopsy series and epidemiological data further supported the notion that secondary bacterial infections were the primary cause of death.

These findings challenged the prevailing belief that influenza alone was responsible for the high mortality rates observed.

Polymicrobial Nature of Fatal Influenza:

During the late 19th and early 20th centuries, it was widely believed that fatal influenza infections involved a combination of an inciting agent with low pathogenicity (such as Bacillus influenzae) and known pneumopathogenic bacteria.

Measles epidemics in US Army training camps in 1917-1918, where streptococcal or pneumococcal pneumonia led to most deaths, mirrored the pattern observed during the influenza pandemic.

Pathologists of that era considered these two events as manifestations of the same disease: epidemic bacterial pneumonia triggered by prevalent respiratory tract agents.

Revisiting the Viral-Bacterial Interplay:

The question of whether severe influenza-associated pneumonia was primarily viral or a result of viral-bacterial coinfection was debated in 1918-1919, with no definitive resolution.

Subsequent experimental studies demonstrated the synergistic effects of influenza viruses and pneumopathic bacteria in animals. Influenza viruses alone were non-pathogenic, but when combined with bacteria, they induced severe disease.

These findings indicated that the interplay between the infecting virus and secondary bacterial infections influenced the pathogenicity of influenza-associated pneumonia.

Mechanisms of Coinfection Pathogenicity:

Recent research has identified possible mechanisms by which coinfection with influenza viruses and bacteria affects pathogenicity.

These mechanisms include viral neuraminidase-induced exposure of bacterial adherence receptors, bacterial neuraminidase-induced upregulation of influenza infection, and the effects of various immune responses.

The influenza virus’s ability to damage the respiratory epithelium and create an environment favorable for bacterial growth further contributes to the development of secondary bacterial pneumonia.

Implications for Modern Pandemic Preparedness:

Studying the role of bacterial lung infections in influenza-related mortality has become more challenging in the modern era due to the widespread use of antibiotics and intensive care treatments.

Nevertheless, the importance of bacterial pneumonia in pandemic planning should not be underestimated. In addition to antiviral drugs and vaccines, preventing, diagnosing, and treating bacterial pneumonia should be prioritized. Stockpiling antibiotics and bacterial vaccines can help mitigate the impact of secondary bacterial infections during future pandemics.

Factors Affecting Morbidity and Mortality:

Comparing the 1918 pandemic with subsequent outbreaks reveals differences in the spectrum and extent of secondary bacterial pneumonia. The prevalence of different bacteria causing pneumonia changed over time, possibly influenced by antibiotic use, vaccination practices, and demographic and social factors.

Factors such as an aging population, nursing home residents, and individuals with underlying health conditions may alter the profile of morbidity and mortality during future pandemics.

Looking Ahead:

While the viral etiology and timing of the next influenza pandemic remain unpredictable, understanding the lessons from past pandemics is essential for preparedness efforts.

If a future pandemic is caused by a human-adapted strain with increased pathogenicity, the role of bacterial pneumonia in severe cases should not be overlooked.

Vigilance in monitoring the emergence and spread of pneumopathogenic bacteria, as well as continued research into the viral-bacterial interplay, will help us stay one step ahead in mitigating the impact of future pandemics.

Finding Balance: Mask Usage and Overall Health

The historical context of bacterial pneumonia during the 1918 influenza pandemic highlights the intricate relationship between viral and bacterial infections.

Recent studies (sourced below) have raised concerns about certain types of masks, particularly cloth masks, which have been linked to increased respiratory infections among healthcare workers.

While masks are essential in curbing the spread of respiratory illnesses like influenza, we must ensure that our preventive measures do not inadvertently cause harm.

Striking a balance between public health mandates and individual well-being is paramount, considering the evolving understanding of these issues. It is essential to critically evaluate the effectiveness of mask usage in different contexts.

Wearing masks outdoors in the fresh air, for example, may be counterproductive as it can create a moist environment that collects bacterial particles, potentially leading to infections.

While masks play a crucial role in certain settings, such as crowded indoor spaces or areas with a high risk of transmission, we must also recognize the limitations and potential risks associated with their indiscriminate use.

It is imperative to differentiate between situations where mask-wearing is beneficial and where it may not be necessary or even counterproductive.

In our pursuit of public health, we must consider the broader picture of overall health:

  • Patient outcomes are influenced by various factors, including underlying health conditions and chronic diseases.
  • By prioritizing public health initiatives that prevent chronic diseases, promote general well-being, and enhance immune resilience, we can address the root causes of infections and mitigate their impact.
  • To navigate the complexities of viral and bacterial infections effectively, we need a comprehensive approach that combines evidence-based preventive measures, individual responsibility, and a focus on long-term health promotion.
  • By fostering open dialogue, supporting scientific research, and strengthening public health infrastructure, we can better prepare for future pandemics.

We must remain vigilant in our efforts to combat infectious diseases, ensuring that our preventive measures are effective and do not inadvertently contribute to further harm. By adopting an informed and nuanced approach to mask usage, considering the specific context and balancing individual well-being with public health objectives, we can build a healthier and more resilient society.

Simultaneously, by placing a strong emphasis on public health initiatives that prevent chronic diseases and promote overall health, we can mitigate the impact of both viral and bacterial infections.

Let us strive for a future where our actions align with the goal of preserving and enhancing public health while minimizing unintended consequences.


  • David M.. Morens and others, Predominant Role of Bacterial Pneumonia as a Cause of Death in Pandemic Influenza: Implications for Pandemic Influenza Preparedness, The Journal of Infectious Diseases, Volume 198, Issue 7, 1 October 2008, Pages 962–970,
  • University of New South Wales. “Cloth masks: Dangerous to your health?.” ScienceDaily. ScienceDaily, 22 April 2015. <>

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