Pneumonia is a lung infection causing inflammation of the air sacs (alveoli). These sacs fill with pus and fluid, making breathing difficult and painful. This condition represents a significant global health concern, affecting people of all ages and remaining a leading infectious cause of death worldwide. While symptoms can appear similar across cases, pneumonia can be caused by various microscopic organisms.
Bacterial Causes of Pneumonia
Bacteria are a common cause of pneumonia, particularly in adults. Streptococcus pneumoniae (pneumococcus) is the most common bacterial cause of community-acquired pneumonia. It often resides harmlessly in the upper respiratory tract but can cause infection when the immune system is weakened.
Another significant bacterium is Haemophilus influenzae, which can cause a range of infections, including pneumonia. It is spread through respiratory droplets from coughing or sneezing, and while the Hib vaccine has reduced its prevalence in some regions, it remains a cause of lower respiratory tract infections. Mycoplasma pneumoniae causes “walking pneumonia,” a milder form where symptoms are less severe, allowing individuals to continue daily activities. This bacterium is contagious, spreading through respiratory droplets, with symptoms appearing gradually over one to four weeks.
Chlamydophila pneumoniae represents another bacterium that can cause atypical pneumonia, often presenting with milder symptoms similar to a chest cold. Legionella pneumophila is responsible for Legionnaires’ disease, a severe form of pneumonia. This bacterium is distinct as it is not typically passed from person to person but is often linked to contaminated water sources like cooling towers and whirlpool spas.
Viral Causes of Pneumonia
Viruses are also significant contributors to pneumonia, especially in children. Influenza viruses (the flu) can directly cause pneumonia or weaken the respiratory system, making individuals susceptible to secondary bacterial infections. Flu-related pneumonia can be severe and is a common cause of acute respiratory distress syndrome in adults.
Respiratory Syncytial Virus (RSV) frequently causes pneumonia, particularly in infants and young children. It spreads through respiratory droplets and can lead to serious illness in vulnerable populations. Adenoviruses and parainfluenza viruses are additional viral agents capable of causing pneumonia. These viruses are common causes of respiratory infections and can progress to lung inflammation.
SARS-CoV-2, the virus causing COVID-19, has been a significant cause of pneumonia. It can lead to lung inflammation and fluid buildup, with severe cases potentially progressing to acute respiratory distress syndrome. COVID-19 pneumonia symptoms vary, but breathing difficulties commonly indicate progression.
Fungal and Other Less Common Causes
Fungi are a less common but important cause of pneumonia, typically affecting individuals with weakened immune systems. Pneumocystis jirovecii is a notable fungal cause, frequently seen in immunocompromised individuals like those with HIV/AIDS or organ transplant recipients. This opportunistic infection can be severe and life-threatening.
Other fungi like Histoplasma capsulatum, Coccidioides immitis, and Cryptococcus neoformans can also cause pneumonia. These fungi are often geographically specific, found in soil or associated with bird and bat droppings. Inhalation of their spores can lead to lung infections, which may present with flu-like symptoms. While rare, parasitic infections can also cause pneumonia in specific circumstances, though they are not a primary focus due to their infrequency in the general population.
Why Identifying the Microbe Matters
Identifying the specific microbe causing pneumonia is important for guiding effective treatment. Bacterial pneumonia is treated with antibiotics, while viral infections do not respond. Fungal pneumonia requires antifungal drugs. A correct diagnosis prevents ineffective treatments.
Using the wrong medication, such as antibiotics for a viral infection, is ineffective and contributes to antibiotic resistance. Knowing the causative agent can also have broader public health implications, aiding in tracking outbreaks and understanding disease patterns. Furthermore, identifying the specific microbe helps in predicting the expected course of the illness and tailoring management strategies for improved patient outcomes.