Pneumonia is an infection of the lung that can be caused by nearly any class of organism known to cause human infections. These include bacteria, amoebae, viruses, fungi, and parasites. Pneumonia may also result from noninfectious causes, such as inhalation of food, liquids, gases, or dust. Pneumonia often develops as a complication of a preexisting condition or infection or when a patient's immune system is weakened by a condition such as a simple viral respiratory tract infection or by influenza. Pneumonia and influenza together are ranked as the eighth leading cause of death in the United States, with pneumonia accounting for most of those deaths. In the elderly, pneumonia is the fourth-leading cause of death and the leading infectious cause of death. In 2017, approximately 55,600 people in the United States died of pneumonia and influenza.

When a person has pneumonia, the air sacs in the lungs become filled with pus and other liquids, and oxygen transfer from the lungs to the bloodstream is inhibited. Without sufficient oxygen, body cells cannot function properly. Lobar pneumonia affects a section (lobe) of a lung while bronchial pneumonia affects patches throughout both lungs.

To better understand pneumonia, it is important to understand the basic anatomic features of the respiratory system. The human respiratory system begins at the nose and mouth, where air is breathed in (inspired) and out (expired). The air tube extending from the nose is called the nasopharynx. The tube carrying air breathed in through the mouth is called the oropharynx. The nasopharynx and the oropharynx merge into the larynx. The oropharynx also carries swallowed substances, including food, water, and salivary secretion, which must pass into the esophagus and then the stomach. The larynx is protected by a trapdoor called the epiglottis. The epiglottis prevents substances that have been swallowed, as well as substances Page 852  |  Top of Articlethat have been regurgitated (thrown up), from heading down into the larynx and toward the lungs.

X-ray showing pneumonia in the rightlung (© SPL/Photo Researchers, Inc. Reproduced by permission.) (© SPL/Photo Researchers, Inc. Reproduced by permission.)

A useful method of picturing the respiratory system is to imagine an upside-down tree. The larynx flows into the trachea, which is the tree trunk, and thus the broadest part of the respiratory tree. The trachea divides into two tree limbs, the right and left bronchi. Each one of these branches off into multiple smaller bronchi, which course through the tissue of the lung. Each bronchus divides into tubes of smaller and smaller diameter, finally ending in the terminal bronchioles. The air sacs of the lung, in which oxygen-carbon dioxide exchange actually takes Page 853  |  Top of Articleplace, are clustered at the ends of the bronchioles like the leaves of a tree. They are called alveoli.

The tissue of the lung that serves only a supportive role for the bronchi, bronchioles, and alveoli is called the lung stroma (or lung parenchyma).

The main function of the respiratory system is to provide oxygen, the most important energy source for the body's cells. Inspired air (the air we breathe in) contains the oxygen, and travels down the respiratory tree to the alveoli. The oxygen moves out of the alveoli and is sent into circulation throughout the body as part of the red blood cells. The oxygen in the inspired air is exchanged within the alveoli for the waste product of human metabolism, carbon dioxide. The air we breathe out contains the gas called carbon dioxide. This gas leaves the alveoli during expiration. To restate this exchange of gases simply, we breathe in oxygen, we breathe out carbon dioxide.

The healthy human lung is sterile. There are no normally resident bacteria or viruses (unlike the upper respiratory system and parts of the gastrointestinal system, where bacteria dwell even in a healthy state). There are multiple safeguards along the path of the respiratory system. These are designed to keep invading organisms from leading to infection.

The first line of defense includes the hair in the nostrils, which serves as a filter for large particles. The epiglottis is a trapdoor of sorts, designed to prevent food and other swallowed substances from entering the larynx and then trachea. Sneezing and coughing, both provoked by the presence of irritants within the respiratory system, help to clear such irritants from the respiratory tract.

Mucus, produced through the respiratory system, also serves to trap dust and infectious organisms. Tiny hair like projections (cilia) from cells lining the respiratory tract beat constantly. They move debris trapped by mucus upward and out of the respiratory tract. This mechanism of protection is referred to as the mucociliary escalator.

Cells lining the respiratory tract produce several types of immune substances which protect against various organisms. Other cells (called macrophages) along the respiratory tract actually ingest and kill invading organisms.

The organisms that cause pneumonia, then, are usually carefully kept from entering the lungs by virtue of these host defenses. However, when an individual encounters a large number of organisms at once, the usual defenses may be overwhelmed, and infection may occur. This can happen either by inhaling contaminated air droplets, or by aspiration of organisms inhabiting the upper airways.

In addition to exposure to sufficient quantities of causative organisms, certain conditions may make an individual more likely to become ill with pneumonia.

Cigarette smoke, inhaled directly by a smoker or secondhand by a innocent bystander, interferes significantly with ciliary function, as well as inhibiting macrophage function, thus predisposing in individual to pneumonia.

Stroke, seizures, alcohol, and various drugs interfere with the function of the epiglottis. This leads to a leaky seal on the trapdoor, with possible contamination by swallowed substances and/or regurgitated stomach contents. Alcohol and drugs also interfere with the normal cough reflex. This further decreases the chance of clearing unwanted debris from the respiratory tract.

Viruses may interfere with ciliary function, allowing themselves or other microorganism invaders (such as bacteria) access to the lower respiratory tract. One of the most important viruses is HIV (human immunodeficiency virus), the causative virus in AIDS (acquired immunodeficiency syndrome). This virus has resulted in a huge increase in the incidence of pneumonia. Because AIDS results in a general decreased effectiveness of many aspects of the host's immune system, a patient with AIDS is susceptible to all kinds of pneumonia. This includes some previously rare parasitic types which would be unable to cause illness in an individual possessing a normal immune system.

Pneumonia is sometimes a pulmonary condition affecting cancer patients, and may indicate that the cancer is progressing or that the patient has developed a new problem. Both cancer and the therapies used to treat it can injure the lungs or weaken the immune system in ways that make cancer patients especially susceptible to the bacteria, fungi, viruses, and other organisms that cause pneumonia. Tumors and infections can block the patient's airway or limit the lungs' ability to rid themselves of fluid and other accumulated secretions that make breathing difficult. Radiation treatment for breast cancer increases the risk of pneumonia in some patients by weakening lung tissue. Other factors that increase a cancer patient's risk of developing pneumonia include:

Various chronic conditions predispose a person to infection with pneumonia. These include asthma, cystic fibrosis, and neuromuscular diseases which may interfere with the seal of the epiglottis. Esophageal disorders may result in stomach contents passing upward into the esophagus. This increases the risk of aspiration into the lungs of those stomach contents with their resident bacteria. Diabetes, sickle cell anemia, lymphoma, leukemia, and emphysema also predispose a person to pneumonia.

Pneumonia is also one of the most frequent infectious complications of all types of surgery. Many drugs used during and after surgery may increase the risk of aspiration, impair the cough reflex, and cause a patient to underfill their lungs with air. Pain after surgery also discourages a patient from breathing deeply enough, and from coughing effectively.

Certain other conditions can increase the risk of pneumonia. These include the following:

Pneumonia can develop gradually in children after exposure to the causative organism, or it can develop quickly after another illness, reducing the lungs' ability to receive and distribute oxygen. It can be mild and easily cured with antibiotics and rest, or it can be severe and require hospitalization. The onset, duration, and severity of pneumonia depend upon the type of infective organism invading the body and the response of the child's immune system in fighting the infection. Respiratory distress represents 20% of all admissions of children to hospitals, and pneumonia is the underlying cause of most of these admissions.

Bacterial pneumonia develops after the child inhales or aspirates pathogens. Viral pneumonia stems primarily from inhaling infected droplets from the upper airway into the lungs. In neonates, pneumonia may result from colonization of the infant's nasopharynx by organisms that were in the birth canal at the time of delivery.

Pneumonia is one of the common and significant diseases of the elderly, especially those over the age of 70. In general, the elderly are more susceptible to pneumonia than younger people. The elderly are also more likely to be hospitalized for pneumonia and need mechanical ventilation, resulting in a longer hospital stay than younger persons. In addition, many elderly people contract pneumonia while staying in a hospital for other conditions, because their immune systems are often compromised due to the condition that initially required treatment.

The elderly have a less effective mucociliary escalator, as well as changes in their immune system. This causes this age group to be more at risk for the development of pneumonia.

The intensity of symptoms and clinical manifestations of pneumonia are often less in the elderly than in younger patients, thus complicating diagnosis of the disease. The elderly may lose lung capacity as they age, making it harder for them to cough productively. They are also often used to feeling ill so may not recognize new symptoms of illness. Elderly people with pneumonia commonly exhibit acute confusion or delirium and deterioration of base metabolic functions.

In the United States, pneumonia is the eighth-most-common disease leading to death; 2 million Americans develop pneumonia each year, and 40,000–70,000 die from it. Pneumonia is also the most common fatal infection acquired by already hospitalized patients. In developing countries, pneumonia ties with diarrhea as the most common cause of death. According to the Centers for Disease Control and Prevention (CDC), the number of deaths from pneumonia in the United States has declined slightly since 2001, however, even in nonfatal cases, pneumonia is a significant economic burden on the healthcare system. One study estimates that people in the American workforce who develop pneumonia cost employers five times as much in health care as the average worker.

The epidemic of HIV, has resulted in a huge increase in the incidence of pneumonia. Because AIDS results in immune system suppression, individuals with AIDS are highly susceptible to all kinds of pneumonia, including some previously rare parasitic types that would not cause illness in someone with a normal immune system.

Every year in the United States, 2 million people of all ages develop pneumonia, including 4% of all the children in the country. It is the eighth-most-common disease leading to death and the fourth-leading cause of death in the elderly; 40,000 to 70,000 people die from pneumonia each year. The incidence of pneumonia in children younger than one year of age is 35 to 40 per 1,000; 30 to 35 per 1,000 children ages two to four; and 15 per 1,000 children between ages five and nine. Fewer than 10 children in 1,000 over age nine are reported to develop pneumonia.

One-sixth of the 6 million pneumonia cases that develop each year occur primarily in persons aged 65 years and older. Over 90% of all deaths from pneumonia occur in the older population. The incidence of development of pneumonia in the elderly is 20 to 40 illnesses per 1,000 persons for pneumonia acquired in community settings, while the incidence rises to 100 to 250 per 1000 persons in cases acquired in long-term care facilities. An estimated 2.1% of elderly residents in longterm-care facilities at any one time have pneumonia. About $1 billion per year are spent on medical therapy to treat bacterial pneumonia in the elderly.

The list of organisms that can cause pneumonia is very large, and includes nearly every class of infecting organism: viruses, bacteria, bacteria-like organisms, fungi, and parasites. Some organisms are more frequently encountered by specific age groups. In addition, some characteristics of an individual may place him or her at greater risk for infection by particular types of organisms:

Other bacteria that cause pneumonia, especially in institutional settings, include Klebsiella, Pseudomonas aeruginosa, Enterobacter species, Proteus species, Escherichia coli, and other gram-negative bacteria. Strains of anaerobic bacteria can be aspirated into the lungs by the elderly due to conditions associated with aging (such as sedative use or neurological conditions) and cause pneumonia. Haemophilus influenzae is a bacterium that causes pneumonia more frequently in patients with chronic bronchitis.

Pneumonia caused by Mycoplasma pneumoniae is a common cause of pneumonia that is usually not a significant threat to the health of the elderly, as it usually affects people younger than 40. Persons at highest risk for mycoplasma pneumonia are those living or working in crowded areas such as schools and homeless shelters, although many people who contract mycoplasma pneumonia have no identifiable risk factor. Symptoms typical of pneumonia are usually mild and appear over a period of one to three weeks. They may become more severe in some people.

PCP is caused by a fungus, Pneumocystis jiroveci. PCP develops in persons with weakened immune systems from causes such as cancer, chronic use of corticosteroids or other medications that affect the immune system, HIV/AIDS, or solid organ and/or bone marrow transplants. Symptoms of PCP include a mild and dry cough, fever, rapid breathing, and shortness of breath, especially upon exercise or activity. PCP was a rare disease before the AIDS disease developed. This type of pneumonia may also be referred to as pneumocystis pneumonia.

Chemical pneumonia is an unusual type of lung irritation. Although pneumonia usually is caused by a bacterium or virus, in chemical pneumonia, inflammation of lung tissue can be caused by many types of chemicals, including liquids, gases, and small particles, such as dust or fumes. Only a small percentage of pneumonias are caused by chemicals. Some chemicals harm only the lungs; however, some toxic chemicals may affect other organs in addition to the lungs and can result in serious organ damage or death. Aspiration pneumonia is another Page 856  |  Top of Articleform of chemical pneumonia, where oral secretions or stomach contents are aspirated into the lungs. Inflammation develops from the toxic effects of stomach acid and enzymes on lung tissue. Symptoms of chemical pneumonia may include:

Half of all pneumonia cases are caused by viruses, including the influenza virus, parainfluenza virus, adenovirus, rhinovirus, herpes simplex virus, respiratory synctial virus, hantavirus, and cytomegalovirus. Many of these pneumonia infections are mild and may last only a short time. Nevertheless, pneumonia caused by the influenza virus may be severe and occasionally fatal. The symptoms of influenza pneumonia are similar to those of influenza, including fever, dry cough, headache, muscle pain, and weakness. However, within 12 to 36 hours, breathlessness develops, and the coughing increases, with a small amount of mucus produced. Patients have a high fever and may develop blueness of the lips. Eighty percent of deaths in recent influenza epidemics occurred in persons aged 65 and older, mostly due to development of complications such as sepsis or acute respiratory distress syndrome. Viral pneumonia can be further complicated by development of bacterial pneumonia.

Pneumonia is suspected in any patient who has fever, cough, chest pain, shortness of breath, and increased respirations (number of breaths per minute). Fever with a shaking chill is even more suspicious. Many patients cough up clumps of sputum, commonly known as spit. These secretions are produced in the alveoli during an infection or other inflammatory condition. They may appear streaked with pus or blood. Severe pneumonia results in the signs of oxygen deprivation. This includes blue appearance of the nail beds or lips (cyanosis).

The invading organism causes symptoms, in part, by provoking an overly strong immune response in the lungs. In other words, the immune system, which should help fight off infections, kicks into such high gear that it damages the lung tissue and makes it more susceptible to infection. The small blood vessels in the lungs (capillaries) become leaky, and protein-rich fluid seeps into the alveoli. This results in less functional area for oxygen-carbon dioxide exchange. The patient becomes relatively oxygen deprived, while retaining potentially damaging carbon dioxide. The patient breathes faster and faster in an effort to bring in more oxygen and blow off more carbon dioxide.

Mucus production is increased, and the leaky capillaries may tinge the mucus with blood. Mucus plugs actually further decrease the efficiency of gas exchange in the lung. The alveoli fill further with fluid and debris from the large number of white blood cells being produced to fight the infection.

Consolidation, a feature of bacterial pneumonias, occurs when the alveoli, which are normally hollow air spaces within the lung, instead, become solid due to quantities of fluid and debris.

Viral pneumonias and mycoplasma pneumonias do not result in consolidation. These types of pneumonia primarily infect the walls of the alveoli and the stroma of the lung.

Severe acute respiratory syndrome (SARS) is a contagious and potentially fatal disease that first appeared in the form of a multicountry outbreak in early February 2003. Later that month, the CDC began to work with the World Health Organization (WHO) to investigate the cause(s) of SARS and to develop guidelines for infection control. SARS has been described as an “atypical pneumonia of unknown etiology;” by the end of March 2003, the disease agent was identified as a previously unknown coronavirus.

The early symptoms of SARS include a high fever with chills, headache, muscle cramps, and weakness. This early phase is followed by respiratory symptoms, usually a dry cough and painful or difficult breathing. Some patients require mechanical ventilation. The mortality rate of SARS is thought to be about 10%, but no SARS cases have been reported since 2004.

For the most part, diagnosis is based on the patient's report of symptoms, combined with examination of the chest. Listening with a stethoscope will reveal abnormal sounds, and tapping on the patient's back (which should Page 857  |  Top of Articleyield a resonant sound due to air filling the alveoli) may instead yield a dull thump if the alveoli are filled with fluid and debris.

Laboratory diagnosis can be made of some bacterial pneumonias by obtaining a sputum specimen and staining the sputum with special chemicals and looking at it under a microscope. Identification of the specific type of bacteria may require culturing the sputum (using the sputum sample to grow greater numbers of the bacteria in a lab dish).

X-ray examination of the chest may reveal certain abnormal changes associated with pneumonia. Localized shadows obscuring areas of the lung may indicate a bacterial pneumonia, while streaky or patchy appearing changes in the x-ray picture may indicate viral or mycoplasma pneumonia. These changes on x-ray, however, are known to lag in time behind the patient's actual symptoms.

The doctor may do a bronchoscopy (visualizing inside the airway via a scope), or may remove a small piece of lung tissue (transbronchial biopsy) for microscopic examination and cultures. If the patient's condition continues to worsen, the doctor may remove additional lung tissue via thoracic needle biopsy or open lung biopsy, for microscopic analysis and cultures.

Prior to the discovery of penicillin antibiotics, bacterial pneumonia was almost always fatal. Today, antibiotics, especially given early in the course of the disease, are very effective against bacterial causes of pneumonia. Erythromycin and tetracycline improve recovery time for symptoms of mycoplasma pneumonia. They do not, however, eradicate the organisms. Amantadine and acyclovir may be helpful against certain viral pneumonias.

A newer antibiotic named linezolid (Zyvox) is being used to treat penicillin-resistant organisms that cause pneumonia. Linezolid was the first of a new line of antibiotics known as oxazolidinones. Another drug known as ertapenem (Invanz) is reported to be effective in treating bacterial pneumonia.

Patients may also be given fluids and possibly drug therapy to thin mucus secretions (mucolytic agents) or medication to open the airways of the lung (brochodilators). Cough suppressants may be given as well as pain medication and fever-reducing medication. Hospitalized patients often receive oxygen, respiratory therapy, and intravenous antibiotics and fluids.

Pneumonia in cancer patients must be treated promptly in order to speed recovery and prevent complications that could arise if the inflammation were allowed to linger. Treatment always includes bed rest and coughing to expel phlegm and other fluids from the lungs (productive cough). To determine which course of treatment would be most appropriate, a doctor considers when symptoms first appeared, what pattern the illness has followed, and whether cancer or its treatments have diminished the patient's infection-fighting ability (immune response).

Most forms of pneumonia are not reportable diseases, and so are reported to public health agencies only through informal procedures, such as calls from primary healthcare workers. Knowing about the existence of such cases in a community can be of importance to public health agencies because they allow public health workers to follow up on possible sources of the disease and its spread among members of the community. When cases of pneumonia are identified within a community, public health workers collect basic information about the disease characteristics, patients' symptoms, and other relevant information that can be used to decide on the most reasonable treatment to be recommended. That information can also be used to provide information about an pneumonia outbreak to health professionals and to the general public. This type of educational program about pneumonia would normally include possible sources of the disease, methods of transmission, treatment options, and methods of prevention.

Prognosis varies according to the type of organism causing the infection. Recovery following pneumonia with Mycoplasma pneumoniae is nearly 100%. Staphylococcus pneumoniae has a death rate of 30%–40%. Similarly, infections with a number of gram-negative bacteria (such as those in the gastrointestinal tract that can cause infection following aspiration) have a death rate of 25%–50%. Streptococcus pneumoniae (also referred to as pneumococcal pneumonia), the most common organism causing pneumonia, produces a death rate of about 5%. More complications occur in the very young or very old individuals who have multiple areas of the lung infected simultaneously. Individuals with other chronic illnesses (including cirrhosis of the liver, congestive heart failure, individuals without a functioning spleen, and individuals who have other diseases that result in a weakened immune system, experience complications. Patients with immune disorders, various types of cancer, transplant patients, and AIDS patients also experience complications.

The chances of an early recovery (within two to three weeks) from pneumonia are enhanced if the pneumonia is detected early, if the patient has a strong immune system, if the infection has not spread throughout the body, and if the patient is not suffering from other diseases.

Measures that can be taken to prevent pneumonia include frequent washing of hands, elimination of the use of tobacco (which damages the ability of the lungs to withstand infections), and wearing of masks in dusty or moldy areas. Since pneumonia often follows common respiratory infections such as the cold or flu, an important preventive measure is to be alert to any symptoms of respiratory illness that last for more than a few days. The practice of deep breathing for patients recovering in the hospital from various diseases or surgeries is recommended to help prevent them from developing pneumonia.

Because many bacterial pneumonias occur in patients who are first infected with the influenza virus (the flu), yearly vaccination against influenza can decrease the risk of pneumonia for certain patients. This is particularly true of the elderly and people with chronic diseases (such as asthma, cystic fibrosis, other lung or heart diseases, sickle cell disease, diabetes, kidney disease, and forms of cancer).

A specific vaccine against Streptococcus pneumoniae is very protective and should also be administered to patients with chronic illnesses.

Patients who have decreased immune resistance are at higher risk for infection with Pneumocystis carinii. They are frequently put on a regular drug regimen of trimethoprim sulfa and/or inhaled pentamidine to avoid pneumocystis pneumonia.

The flu vaccine helps prevent pneumonia caused by influenza viruses. This vaccine must be given yearly to protect against new viral strains.

Additional preventive therapy may be necessary for:

In most cases, a diagnosis of pneumonia is made in a physician's office, a general medical clinic, or emergency room by a primary care practitioner. Children and adolescents with pneumonia are most likely to be diagnosed by their primary care physician or pediatrician.

When patients are hospitalized for pneumonia, good nursing assessment and observation are primary requirements. These include monitoring vital signs, including oxygen saturation (the amount of oxygen circulating in the blood), encouraging the patient to move, breathe deeply, cough, and get out of bed with assistance (if indicated) to facilitate good lung expansion. The nurse should also provide education to the patient about the importance of coughing, breathing deeply, and taking in adequate fluid.

When at home, patients should be encouraged to drink fluids to loosen secretions and bring up phlegm. Both patients and care givers should be made aware of potential drug interactions with other medications that the patient may be taking (for example, warfarin and antibiotics). Regular communication between the physician and the care giver is essential.