Pneumonia is an infection that inflames the air sacs in one or both lungs. The air sacs (alveoli) may fill with fluid or pus, causing symptoms like cough, fever, chills, and difficulty breathing. While anyone can develop this lung infection, it can range in seriousness from mild to potentially life-threatening. The condition can be particularly dangerous for older adults and individuals with underlying health issues.
Bacterial Causes
Bacteria are microorganisms that can cause infection when they enter the lungs. These pathogens can spread to the lungs from the nose, sinuses, or mouth, or be directly inhaled. Once in the lungs, bacteria trigger an immune response, causing inflammation and fluid accumulation in the air sacs, which impairs oxygen exchange.
Streptococcus pneumoniae (pneumococcus) is the most frequent bacterial cause of pneumonia in adults. Other significant bacterial culprits include Haemophilus influenzae, Staphylococcus aureus, and Klebsiella pneumoniae. Staphylococcus aureus, particularly methicillin-resistant Staphylococcus aureus (MRSA), is concerning due to its antibiotic resistance.
Pneumonia acquired outside of a hospital setting is known as community-acquired pneumonia (CAP). Conversely, hospital-acquired pneumonia (HAP) develops in individuals who have been hospitalized for another illness or procedure. HAP can be more serious because it is frequently caused by antibiotic-resistant bacteria, making it more challenging to treat.
Viral Causes
Viruses are microscopic infectious agents that replicate inside living cells. Viral infections are a significant cause of adult pneumonia, often damaging lung tissue and making it more susceptible to inflammation. In some cases, a viral infection can also weaken the lungs, leading to a secondary bacterial infection.
Common viruses that can lead to pneumonia include influenza (flu) viruses, respiratory syncytial virus (RSV), and adenoviruses. Human metapneumovirus is another virus that can cause pneumonia. SARS-CoV-2, the virus responsible for COVID-19, has also emerged as a prominent cause of viral pneumonia, sometimes leading to severe lung complications.
Symptoms of viral pneumonia can initially resemble those of the flu, such as fever, cough, and muscle pain. As the infection progresses, symptoms like increased coughing and shortness of breath may worsen. While many viral pneumonias resolve on their own, some can be severe and require medical attention.
Other Microbial Causes
Beyond common bacteria and viruses, other microorganisms can cause pneumonia, particularly in specific circumstances. Fungal pneumonia, for instance, is more common in individuals with weakened immune systems or those exposed to certain environmental fungi. Examples include Pneumocystis jirovecii, and Histoplasma capsulatum or Coccidioides, found in specific geographic regions.
Another category involves “atypical” bacteria, which cause pneumonia with different symptoms and antibiotic responses than typical bacterial infections. Mycoplasma pneumoniae is a well-known example, often causing what is referred to as “walking pneumonia” due to its milder symptoms. Chlamydia pneumoniae is another atypical bacterium that can cause respiratory infections, including pneumonia.
Non-Infectious Causes
Pneumonia can also result from non-infectious factors. One such condition is aspiration pneumonia, which occurs when foreign material is inhaled into the lungs instead of being swallowed. This material can include food, liquids, vomit, or even saliva.
When these substances enter the airways, they can directly irritate the lung tissue, leading to inflammation. Oral bacteria present in the aspirated material can also introduce infection. Another non-infectious cause is chemical pneumonia, also known as toxic pneumonia. This type results from inhaling toxic gases, fumes, or dust, or from aspirating certain chemicals like petroleum products. These substances directly damage lung tissue, leading to inflammation and compromised function.