Pneumonia can be caused by dozens of different microorganisms spanning three broad categories: bacteria, viruses, and fungi. Which one is responsible in a given case depends heavily on the patient’s age, immune status, geographic location, and whether the infection was picked up in everyday life or inside a hospital. In children hospitalized with community-acquired pneumonia in the United States, viruses account for the majority of identified cases, while bacteria dominate in older adults and in hospital settings. Fungi, though less common overall, cause severe disease in people with weakened immune systems and in anyone who inhales spores from certain soils.
The Bacterial Culprits
Streptococcus pneumoniae (the pneumococcus) has been the single most recognized bacterial cause of pneumonia for well over a century, and it remains a leading cause of community-acquired pneumonia worldwide. This bacterium normally lives harmlessly in the nose and throat. It becomes dangerous when it migrates into the lungs, where its polysaccharide capsule helps it evade the immune system by suppressing inflammatory signals in lung tissue and shielding it from being engulfed by immune cells.1PubMed Central. The pneumococcal polysaccharide capsule and pneumolysin differentially affect CXCL8 and IL-6 release from cells of the upper and lower respiratory tract The result is a rapid takeover of alveolar air sacs, filling them with fluid and inflammatory debris and producing the classic symptoms of fever, cough with rust-colored sputum, and chest pain.
Mycoplasma pneumoniae is a frequent cause of so-called “walking pneumonia,” a milder form that often hits school-age children and young adults. Unlike the pneumococcus, Mycoplasma lacks a cell wall entirely, which makes it naturally resistant to penicillin-type antibiotics. It produces a toxin called CARDS toxin that is rapidly taken up by human airway cells through a specific uptake pathway, contributing to the inflammation and airway damage that follow infection.2PubMed Central. Mycoplasma pneumoniae CARDS toxin is internalized via clathrin-mediated endocytosis In a large U.S. study of children hospitalized with pneumonia from 2010 to 2012, Mycoplasma was actually the most commonly identified bacterial pathogen, found in about 8% of cases, compared to roughly 4% for S. pneumoniae.3Clinical and Experimental Pediatrics. Epidemiology and surveillance implications of community-acquired pneumonia in children
Staphylococcus aureus, including its drug-resistant form MRSA, and Pseudomonas aeruginosa are less common in everyday community pneumonia but carry outsized importance because they are harder to treat. In one hospital-system analysis of community-acquired pneumonia, MRSA and Pseudomonas together accounted for about 3% of the overall cohort but roughly 30% of cases where a specific bacterium was confirmed by culture.4JACEP Open. Methicillin‐resistant Staphylococcus aureus and Pseudomonas aeruginosa community acquired pneumonia: Prevalence and locally derived risk factors in a single hospital system These organisms tend to show up in people with prior hospitalizations, chronic lung disease, or recent antibiotic use.
Other bacteria worth knowing about include Haemophilus influenzae, Legionella pneumophila (the cause of Legionnaires’ disease, often linked to contaminated water systems), and Chlamydophila pneumoniae. In hospital-acquired and ventilator-associated pneumonia, the cast of characters shifts toward gram-negative rods like Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas, many of which carry multiple antibiotic resistance genes.
Viruses and Their Outsized Role in Childhood Pneumonia
The role of viruses in pneumonia has become clearer over the past two decades as molecular testing has improved. In the same large U.S. pediatric study mentioned above, a pathogen was identified in 81% of hospitalized children. Viruses accounted for about two-thirds of those cases, with respiratory syncytial virus (RSV) found in 28%, human rhinovirus in 27%, human metapneumovirus in 13%, and adenovirus in 11%.3Clinical and Experimental Pediatrics. Epidemiology and surveillance implications of community-acquired pneumonia in children In adults, influenza viruses remain a major cause of viral pneumonia, and SARS-CoV-2 demonstrated during the COVID-19 pandemic just how devastating a respiratory virus can be at the population level.
Viral pneumonia damages the lungs differently from bacteria. Respiratory viruses cause extensive cell death in the lung lining through a combination of direct viral action and the immune system’s own inflammatory response. Recovery requires the rapid replacement of damaged tissue by progenitor cells, but growing evidence shows that viral infections can induce long-lasting changes in these progenitor cells that persist well after the infection clears.5PubMed Central. The long reach of influenza and other respiratory viruses: from acute epithelial injury to post-viral lung disease This may help explain why some people experience lingering respiratory symptoms for months after a severe bout of viral pneumonia.
In severe COVID-19, the signature injury pattern in the lungs is diffuse alveolar damage, where the thin air-sac walls become swollen and lined with a glassy material called hyaline membrane.6PubMed Central. COVID-19-Associated Acute Respiratory Distress Syndrome: Lessons from Tissues and Cells Under a microscope, this damage looks essentially identical to what influenza, RSV, or other severe viral infections produce in the lungs, meaning pathologists cannot distinguish COVID lung injury from other viral causes based on tissue appearance alone.7PubMed Central. Diffuse alveolar damage (DAD) resulting from coronavirus disease 2019 Infection is Morphologically Indistinguishable from Other Causes of DAD The shared endpoint highlights that the damage in severe viral pneumonia is largely driven by the body’s inflammatory response, not just the specific virus involved.
When a Virus Opens the Door for Bacteria
One of the most clinically important interactions in pneumonia is bacterial superinfection following a viral illness. Influenza is the best-studied example: a flu infection weakens both the physical barrier of the airway lining and the immune defenses that normally keep bacteria in check. Both the innate first-response system and the adaptive immune system’s antibacterial capabilities are impaired after influenza.8PubMed Central. Influenza and Bacterial Superinfection: Illuminating the Immunologic Mechanisms of Disease Staphylococcus aureus and Streptococcus pneumoniae are the two bacteria most commonly involved in post-influenza superinfections, and this combination contributed substantially to the death toll in both the 1918 and 2009 influenza pandemics.9PubMed Central. Mechanisms of Bacterial Superinfection Post-influenza: A Role for Unconventional T Cells
This is why clinicians pay close attention when a patient with the flu appears to improve and then suddenly worsens several days later. A secondary bacterial pneumonia layered on top of a viral infection often progresses faster and hits harder than either pathogen would on its own. The same pattern has been observed with SARS-CoV-2, where critically ill COVID patients sometimes develop secondary bacterial or fungal lung infections during prolonged ICU stays.
Fungal Pneumonia and Who Is at Risk
Fungal pneumonia falls into two broad buckets: opportunistic infections that target people with compromised immune systems, and endemic infections that can strike anyone who breathes in the wrong spores in the right geography.
Pneumocystis jirovecii is the textbook opportunistic fungal cause of pneumonia. It was historically associated almost exclusively with advanced HIV/AIDS, but its profile has shifted. Cases are now increasingly recognized in people without HIV who are immunosuppressed for other reasons, including organ transplant recipients, people on chemotherapy, and those taking immunosuppressive medications for autoimmune conditions.10PubMed Central. Pneumocystis jiroveci Pneumonia: A Review of Management in Human Immunodeficiency Virus (HIV) and Non-HIV Immunocompromised Patients In a study of 160 HIV-negative patients diagnosed with Pneumocystis pneumonia, solid organ transplant recipients had more severe disease and worse outcomes than other immunocompromised groups.11PubMed. Outcome of Pneumocystis pneumonia in transplant and non-transplant HIV-negative immunocompromised patients Aspergillus species are another major opportunistic concern, particularly in patients with severe neutropenia or those on high-dose steroids.
The endemic fungi are a different story. Coccidioides (which causes Valley fever), Histoplasma, and Blastomyces live in soil and become airborne when dirt is disturbed. You do not need a weakened immune system to catch these; you just need to be in the wrong place at the wrong time. These infections have distinct geographic niches: coccidioidomycosis is concentrated in the desert Southwest and parts of California, histoplasmosis clusters in the Ohio and Mississippi River valleys, and blastomycosis in the upper Midwest and Great Lakes region.12Morbidity and Mortality Weekly Report. Surveillance for Coccidioidomycosis, Histoplasmosis, and Blastomycosis — United States, 2019 Among older adults, the incidence differences are stark: coccidioidomycosis ran at about 15 cases per 100,000 person-years in the West, while histoplasmosis and blastomycosis were highest in the Midwest at roughly 6 and 1 per 100,000, respectively.13PubMed Central. Geographic Distribution of Endemic Fungal Infections among Older Persons, United States
Aspiration Pneumonia and Mixed Infections
Not all pneumonia starts with inhaling a single pathogen from the outside. Aspiration pneumonia develops when material from the mouth or stomach enters the lungs. When someone with swallowing difficulties aspirates saliva or food particles, the bacteria normally living in the mouth, including anaerobes and streptococci, can seed the lung and cause a true bacterial pneumonia. If stomach contents are aspirated instead, the initial injury is a chemical burn from gastric acid rather than an infection, though bacteria can take hold afterward.14Australian Prescriber. Aspiration pneumonia and pneumonitis
Aspiration events are extremely common in people with neurological conditions, dementia, heavy sedation, or alcohol intoxication. Interestingly, recent research on the lung microbiome in critically ill patients found that a clinical diagnosis of macro-aspiration does not produce a distinct microbial signature compared to other ICU patients. What researchers did find is that patients with pneumonia, regardless of aspiration history, showed a consistent shift in their lung microbial communities: a depletion of the harmless anaerobic bacteria normally present in the respiratory tract, matched by an overgrowth of the types of bacteria implicated in lung infections.15iScience. What Microbes Cause Pneumonia? Bacteria, Viruses, and Fungi This suggests that the microbial disruption in pneumonia is more about an ecosystem collapse in the lungs than about any single invading organism.
Why Identifying the Cause Is So Difficult
One of the frustrating realities of pneumonia is that doctors often cannot pin down which microbe is responsible. Up to 56% of patients diagnosed with community-acquired pneumonia end up labeled as “culture-negative,” meaning standard lab cultures fail to grow a causative organism.16CHEST. Should Multiplex Molecular Panels Be Performed on All Patients With Community Acquired Pneumonia? This happens for several reasons: the patient may have taken antibiotics before samples were collected, the pathogen may be a virus that does not grow on bacterial culture media, or the diagnosis may actually be wrong (the patient has fluid in the lungs from heart failure or an inflammatory reaction rather than an infection).
Newer molecular diagnostic panels that use PCR to detect genetic material from dozens of pathogens simultaneously are changing this picture. In one ICU study, a multiplex PCR panel detected bacteria in about a quarter of specimens that traditional cultures had called negative.17PubMed Central. Performance of a multiplex PCR pneumonia panel for the identification of respiratory pathogens and the main determinants of resistance from the lower respiratory tract specimens of adult patients in intensive care units These panels can also detect viruses and resistance genes in the same test, giving clinicians a much faster and broader view of what they are dealing with. The practical benefit is significant: when a test comes back showing a virus and no bacteria, physicians are more willing to stop unnecessary antibiotics, something they have historically been reluctant to do with a mere negative culture.
Vaccines and the Moving Target of Pneumococcal Disease
Vaccination against Streptococcus pneumoniae has been one of the genuine public health victories of the past few decades. The original seven-valent conjugate vaccine (PCV7), introduced in 2000, dramatically reduced invasive pneumococcal disease caused by its target serotypes. But S. pneumoniae comes in roughly 100 different serotypes, and the vaccine only covered seven. As the vaccine suppressed those seven, non-vaccine serotypes began filling the ecological niche, a phenomenon called serotype replacement.18PubMed Central. Serotype replacement in disease after pneumococcal vaccination Among asymptomatic carriers, the total rate of pneumococcal colonization barely changed; different serotypes simply took over.
The expanded 13-valent vaccine (PCV13) broadened coverage and drove down disease from its targeted serotypes in turn, but the replacement pattern has continued. Data from Ontario, Canada, covering 2007 through 2022 showed that while invasive disease from PCV13 serotypes declined, cases caused by serotypes covered only in newer, broader vaccines (PCV15 and PCV20) and by non-vaccine serotypes rose.19Open Forum Infectious Diseases. Invasive Pneumococcal Disease Epidemiology and Serotype Replacement After the Introduction of the 13-Valent Pneumococcal Conjugate Vaccine in Ontario, Canada, 2007–2022 This is not a failure of vaccination. The net effect has still been a large reduction in severe pneumococcal disease. But it does mean that the bacterium is a moving target, and vaccine formulations will likely need periodic updating, somewhat analogous to the annual reformulation of flu vaccines. Newer 20-valent and even broader-valent vaccines are already in use or in development to stay ahead of the shift.20Scientific Reports. Divergent serotype replacement trends and increasing diversity in pneumococcal disease in high income settings reduce the benefit of expanding vaccine valency
Antibiotic Resistance and the Treatment Challenge
Treatment of bacterial pneumonia relies heavily on antibiotics, and the rising tide of antimicrobial resistance is making that job harder. In intensive care settings, pneumonia caused by multidrug-resistant bacteria is one of the main clinical challenges, requiring combination therapy and sometimes novel antibiotic molecules to achieve adequate treatment.21PubMed. Risk stratification and treatment of ICU-acquired pneumonia caused by multidrug-resistant/extensively drug-resistant/pandrug-resistant bacteria Gram-negative bacteria like Pseudomonas, Klebsiella, and Acinetobacter are particularly problematic because some strains have accumulated resistance to nearly every available drug class. New antibiotics targeting these gram-negative pathogens have reached the market in recent years, though the pipeline remains thin relative to the pace at which resistance spreads.22F1000Research. Current trends in the treatment of pneumonia due to multidrug-resistant Gram-negative bacteria
Fungi present their own resistance headaches. The number of effective antifungal drug classes is much smaller than for antibiotics, essentially three major classes, and resistance is rising across species of Candida, Aspergillus, Cryptococcus, and Pneumocystis.23PubMed Central. Drug-Resistant Fungi: An Emerging Challenge Threatening Our Limited Antifungal Armamentarium Drug-resistant fungal strains are increasingly showing up in clinical settings after evolving in the environment, likely under selective pressure from agricultural antifungal use. For a patient with invasive fungal pneumonia and a resistant organism, treatment options can be alarmingly few.
Why Older Adults Are Hit Hardest
Pneumonia has long been called “the old man’s friend” for its role as a common final illness in elderly patients, and the biological reasons go well beyond general frailty. As the immune system ages, a cascade of specific deficiencies accumulates. The cilia lining the airways slow down, reducing the ability to sweep out inhaled pathogens. A key protein that holds lung lining cells together, cadherin-1, is produced at lower levels, weakening the physical barrier. Alveolar macrophages, the immune cells that serve as first responders in the lung, become less responsive and lose some of their ability to sense and clear viruses.24European Respiratory Review. Immunosenescence and susceptibility to respiratory viruses: a state-of-the-art review This is why influenza and COVID-19 cause disproportionately severe pneumonia in older adults: the aged lung is slower to detect the virus, slower to mount a coordinated defense, and slower to repair itself afterward.
The combination of weakened barriers and sluggish immune surveillance also makes older adults more vulnerable to bacterial superinfection after a viral illness and to aspiration pneumonia from age-related swallowing difficulties. These overlapping vulnerabilities explain why pneumonia prevention in older populations focuses on multiple fronts simultaneously: pneumococcal and influenza vaccines, COVID boosters, swallowing assessments for those with neurological decline, and prompt treatment of any respiratory infection before it spirals.
Climate Change and the Shifting Geography of Fungal Pneumonia
The geographic boundaries of endemic fungal infections are not fixed. Over the past few decades, cases of histoplasmosis, coccidioidomycosis, and blastomycosis have been reported in areas previously considered non-endemic.25PubMed Central. Re-drawing the Maps for Endemic Mycoses Climate change is a major driver: as arid regions expand and temperatures rise, the soil fungi that cause these diseases can survive in new territories, exposing populations that have no prior immunological experience with these organisms.26PubMed. Pulmonary fungal infections in the age of biologics and climate change
Coccidioidomycosis, or Valley fever, illustrates this most dramatically. Modeling work projects that under a high-warming scenario, the area of the U.S. where Valley fever is endemic could more than double by 2100, the number of affected states could grow from 12 to 17, and annual cases could rise by 50%. The endemic zone would push north into currently dry western states like Idaho, Wyoming, Montana, and the Dakotas.27PubMed Central. Expansion of Coccidioidomycosis Endemic Regions in the United States in Response to Climate Change For physicians in those regions, Valley fever is not yet on the diagnostic radar, which means early cases may be misdiagnosed as bacterial pneumonia and treated with unnecessary antibiotics. The CDC surveillance data already shows that blastomycosis has turned up in New York, a state previously thought to be outside its range.12Morbidity and Mortality Weekly Report. Surveillance for Coccidioidomycosis, Histoplasmosis, and Blastomycosis — United States, 2019 The textbook maps that medical students learned a generation ago are already out of date, and the mismatch between where doctors think these infections exist and where they actually exist is a growing diagnostic blind spot.