Pneumonia is an infection that inflames the air sacs in one or both lungs. The air sacs may fill with fluid or pus (purulent material), causing cough with phlegm or pus, fever, chills, and difficulty breathing. A variety of organisms, including bacteria, viruses and fungi, can cause pneumonia.
Pneumonia can range in seriousness from mild to life-threatening It is most serious for infants and young children, people older than age 65, and people with health problems or weakened immune systems.
Bacteria and viruses are the primary causes of pneumonia. When a person breathes pneumonia-causing germs into his lungs and his body’s immune system cannot otherwise prevent entry, the organisms settle in small air sacs called alveoli and continue multiplying. As the body sends white blood cells to attack the infection, the sacs become filed with fluid and pus – causing pneumonia.
Pneumonia has bacterial, viral, fungal, and other primary causes. A summary is provided below.
Streptococcus pneumoniae is the most common cause of bacterial pneumonia. People who suffer from chronic obstructive pulmonary disease (COPD) or alcoholism most often get pneumonia from Klebsiella pneumoniae and Hemophilus influenzae. Atypical pneumonia, a type of pneumonia that typically occurs during the summer and fall months, is caused by the bacteria Mycoplasma pneumoniae.
People who have Legionnaire’s disease caused by the bacteriumLegionella pneumoniae (often found in contaminated water supplies and air conditioners) may also develop pneumonia as part of the overall infection. Another type of bacteria responsible for pneumonia is called Chlamydia pneumoniae. Pneumocystis carinii pneumonia is a form of pneumonia that usually affects both lungs and is found in patients with weakened or compromised immune systems from such conditions as cancer and HIV/AIDS and those treated with TNF (tumor necrosis factor) for rheumatoid arthritis.
Viral pneumonias are pneumonias that do not typically respond to antibiotic treatment (in contrast to bacterial pneumonias). Adenoviruses, rhinovirus, influenza virus (flu), respiratory syncytial virus (RSV), and parainfluenza virus are all potential causes of viral pneumonia.
Histoplasmosis, coccidiomycosis, blastomycosis, aspergillosis, and cryptococcosis are fungal infections that can lead to fungal pneumonia. These types of pneumonias are relatively infrequent in the United States.
Nosocomial and others
Organisms that have been exposed to strong antibiotics and have developed resistance are called nosocomial organisms. If they enter the lungs, a person may develop nosocomial pneumonia. Resistant bacteria are often found in nursing homes and hospitals. An example is MRSA, or methicillin-resistant Staph aureus, which can cause skin infections as well as pneumonia. Similarly, outbreaks of the H5N1 influenza (bird flu) virus and severe acute respiratory syndrome (SARS) have resulted in serious pneumonia infections. Anthrax, plague, and tularemia also may cause pneumonia, but their occurrences are rare.
The causes for the development of pneumonia are extrinsic or intrinsic, and various bacterial causes are noted. Extrinsic factors include exposure to a causative agent, exposure to pulmonary irritants, or direct pulmonary injury. Intrinsic factors are related to the host. Loss of protective upper airway reflexes allows aspiration of contents from the upper airways into the lung. Various causes for this loss include altered mental status due to intoxication and other metabolic states and neurologic causes, such as stroke and endotracheal intubation.
Bacteria from the upper airways or, less commonly, from hematogenous spread, find their way to the lung parenchyma. Once there, a combination of factors (including virulence of the infecting organism, status of the local defenses, and overall health of the patient) may lead to bacterial pneumonia. The patient may be made more susceptible to infection because of an overall impairment of the immune response (eg, human immunodeficiency virus [HIV] infection, chronic disease, advanced age) and/or dysfunction of defense mechanisms (eg, smoking,chronic obstructive pulmonary disease [COPD], tumors, inhaled toxins, aspiration). Poor dentition or chronic periodontitis is another predisposing factor.
Thus, during pulmonary infection, acute inflammation results in the migration of neutrophils out of capillaries and into the air spaces, forming a marginated pool of neutrophils that is ready to respond when needed. These neutrophils phagocytize microbes and kill them with reactive oxygen species, antimicrobial proteins, and degradative enzymes. They also extrude a chromatin meshwork containing antimicrobial proteins that trap and kill extracellular bacteria, known as neutrophil extracellular traps (NETs). Various membrane receptors and ligands are involved in the complex interaction between microbes, cells of the lung parenchyma, and immune defense cells.
General mechanisms of increased virulence include the following:
- Genetic flexibility allowing the development of resistance to various classes of antibiotics
- Flagellae and other bacterial appendages that facilitate spread of infection
- Capsules resistant to attack by immune defense cells and that facilitate adhesion to host cells
- Quorum sensing systems allow coordination of gene expression based on complex cell-signaling for adaptation to the local cellular environment
- Iron scavenging
The following are examples of organism-specific virulence factors:
- Streptococcus pneumoniae – Pneumolysin, a multifunctional virulence factor, is cytotoxic to respiratory epithelium and endothelium by disrupting pulmonary tissue barriers. This factor directly inhibits immune and inflammatory cells and activates complement, decreasing the clearance of the bacteria from the lung.
- Pseudomonas aeruginosa – Pili play important role in the attachment to host cells. A type III secretion system allows injection of toxins into host cells.
Deficits in various host defenses and an inability to mount an appropriate acute inflammatory response can predispose patients to infection, as follows:
- Deficits in neutrophil quantity, as in neutropenia
- Deficits in neutrophil quality, as in chronic granulomatous disease
- Deficiencies of complement
- Deficiencies of immunoglobulins
With the recent H1N1 influenza virus pandemic, it is important to address the role that viral infection can have in bacterial pneumonia.
The association between infection with influenza virus and subsequent bacterial pneumonia became particularly apparent following the 1918 influenza pandemic, during which approximately 40-50 million people died. Historical investigations and current researchers argue that the vast majority of pulmonary-related deaths from past pandemic influenza viruses, most notably the pandemic of 1918, ultimately resulted from bacteriologic secondary or coinfection and poorly understood interactions between the infecting viral and bacterial organisms.Although influenza virus is the most commonly thought of agent in this co-infective context, other respiratory viruses, such as respiratory syncytial virus (RSV), parainfluenza viruses, adenovirus, and rhinoviruses, may also predispose to secondary bacterial infection.
The classic explanation behind the viral-bacterial interplay focuses on the disruption of the respiratory epithelium by the virus, providing an opportinistic environment for bacterial infection. However, evidence depicts much more complex and possibly synergistic interactions between viruses and bacteria, including alteration of pulmonary physiology, downregulation of the host immune defense, changes in expression of receptors to which bacteria adhere, and enhancement of the inflammatory process.
The signs and symptoms of pneumonia vary from mild to severe, depending on factors such as the type of germ causing the infection, and your age and overall health. Mild signs and symptoms often are similar to those of a cold or flu, but they last longer.
Signs and symptoms of pneumonia may include:
- Fever, sweating and shaking chills
- Cough, which may produce phlegm
- Chest pain when you breathe or cough
- Shortness of breath
- Nausea, vomiting or diarrhea
Newborns and infants may not show any sign of the infection. Or they may vomit, have a fever and cough, appear restless or tired and without energy, or have difficulty breathing and eating.
People older than age 65 and people in poor health or with a weakened immune system may have a lower than normal body temperature. Older people who have pneumonia sometimes have sudden changes in mental awareness.
A pneumonia diagnosis usually begins with a physical exam and a discussion about your symptoms and medical history. A doctor may suspect pneumonia if they hear coarse breathing, wheezing, crackling sounds, or rumblings when listening to the chest through a stethoscope.
Chest x-rays and blood tests may be ordered to confirm a pneumonia diagnosis. A chest x-ray can confirm pneumonia and determine its location and extent in the lungs. Blood tests measure white blood cell count to determine the severity of pneumonia and can be used to determine whether the infection is bacterial, viral, fungal, etc. An analysis of sputum also can be used to determine the organism that is causing the pneumonia.
A more invasive diagnostic tool is the bronchoscopy – a procedure whereby the patient is under anesthesia and a thin, flexible, and lighted tube is inserted into the nose or mouth to directly examine the infected parts of the lung.
Treatment for pneumonia involves curing the infection and preventing complications. People who have community-acquired pneumonia usually can be treated at home with medication. Although most symptoms ease in a few days or weeks, the feeling of tiredness can persist for a month or more.
Specific treatments depend on the type and severity of your pneumonia, your age and your overall health. The options include:
- Antibiotics. These medicines are used to treat bacterial pneumonia. It may take time to identify the type of bacteria causing your pneumonia and to choose the best antibiotic to treat it. If your symptoms don’t improve, your doctor may recommend a different antibiotic.
- Fever reducers. These include drugs such as aspirin, ibuprofen (Advil, Motrin IB, others) and acetaminophen (Tylenol, others).
- Cough medicine. This medicine may be used to calm your cough so that you can rest. Because coughing helps loosen and move fluid from your lungs, it’s a good idea not to eliminate your cough completely.
You may need to be hospitalized if:
- You are older than age 65
- You become confused about time, people or places
- Your nausea and vomiting prevent you from keeping down oral antibiotics
- Your blood pressure drops
- Your breathing is rapid
- You need breathing assistance
- Your temperature is below normal
- Your heart rate is below 50 or higher than 100
You may be admitted to the intensive care unit if you need to be placed on a breathing machine (ventilator) or if your symptoms are severe.
Children may be hospitalized if they:
- Are younger than age 2 months
- Are excessively sleepy
- Have trouble breathing
- Have low blood oxygen levels
- Appear dehydrated
- Have a lower than normal temperature