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pneumonia (no͝omōnˈyə), acute infection of one or both lungs that can be caused by a bacterium, usually Streptococcus pneumoniae (also called pneumococcus; see streptococcus), or by a virus, fungus, or other organism. The causal organisms reach the lungs through the respiratory passages. Usually an upper respiratory infection precedes the disease. Alcoholism, extreme youth or age, debility, immunosuppressive disorders and therapy, and compromised consciousness are predisposing factors. When one or more entire lobes of the lung are involved, the infection is considered a lobar pneumonia. When the disease is confined to the air spaces adjacent to the bronchi, it is known as bronchopneumonia. Aspiration pneumonia is the pathological consequence of the abnormal entry of fluids, particulate matter, or secretions in the lower airways.

The symptoms of pneumonia are high fever, chills, pain in the chest, difficulty in breathing, cough, and sputum that is pinkish at first and becomes rust-colored as the infection progresses. The skin may turn bluish because the lungs are not sufficiently oxygenating the blood. Complete bed rest and good supportive care are important. Oxygen helps to relieve severe respiratory difficulty.

Immunization for pneumococcal pneumonia is recommended for children under two years old, adults 65 or older, and others at risk. Penicillin is most commonly used to treat pneumococcal pneumonia and other pneumonias caused by bacteria and, with the other antibiotic and sulfa drugs, is responsible for the marked decline since the mid-20th cent. in mortality figures. Nevertheless, pneumonia is still a serious disease, especially in elderly and debilitated persons (who usually acquire bronchopneumonia) or when complicated by bacterial invasion of the bloodstream, membranes of the heart, or the central nervous system.

Viral pneumonia, generally milder than the bacterial form, is the result of lower respiratory infection and has been the cause of more than 90% of deaths for individuals over 65. Pneumocystis carinii pneumonia, which is caused by an organism traditionally thought to be a parasitic protozoan but now suspected to be a fungus, generally only occurs in patients who have AIDS or leukemia or whose immune system is otherwise suppressed.

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An acute or chronic inflammatory disease of the lungs. More specifically when inflammation is caused by an infectious agent, the condition is called pneumonia; when the inflammatory process in the lung is not related to an infectious organism, it is called pneumonitis.

An estimated 45 million cases of infectious pneumonia occur annually in the United States, with up to 50,000 deaths directly attributable to it. Pneumonia is a common immediate cause of death in persons with a variety of underlying diseases. With the use of immunosuppressive and chemotherapeutic agents for treating transplant and cancer patients, pneumonia caused by infectious agents that usually do not cause infections in healthy persons (that is, pneumonia as an opportunistic infection) has become commonplace. Moreover, individuals with acquired immune deficiency syndrome (AIDS) usually die from an opportunistic infection, such as pneumocystis pneumonia or cytomegalovirus pneumonia. Concurrent with the variable and expanding etiology of pneumonia and the more frequent occurrence of opportunistic infections is the development of new antibiotics and other drugs used in the treatment of pneumonia. See Acquired immune deficiency syndrome (AIDS), Opportunistic infections

Bacteria, as a group, are the most common cause of infectious pneumonia, although influenza virus has replaced Streptococcus pneumoniae (Diplococcus pneumoniae) as the most common single agent. Some of the bacteria are normal inhabitants of the body and proliferate to cause disease only under certain conditions. Other bacteria are contaminants of food or water.

Most bacteria cause one of two main morphologic forms of inflammation in the lung. Streptococcus pneumoniae causes lobar pneumonia, in which an entire lobe of a lung or a large portion of a lobe becomes consolidated (firm, dense) and nonfunctional secondary to an influx of fluid and acute inflammatory cells that represent a reaction to the bacteria. This type of pneumonia is uncommon today, usually occurring in people who have poor hygiene and are debilitated. If lobar pneumonia is treated adequately, the inflammatory process may entirely disappear, although in some instances it undergoes a process called organization, in which the inflammatory tissue changes into fibrous tissue, usually rendering that portion of the lung nonfunctional.

The other morphologic form of pneumonia, which is caused by the majority of bacteria, is called bronchopneumonia. In this form there is patchy consolidation of lung tissue, usually around the small bronchi and bronchioles, again most frequently in the lower lobes. This type of pneumonia may also undergo complete resolution if there is adequate treatment, although rarely it organizes.

Viral pneumonia is usually a diffuse process throughout the lung and produces a different type of inflammatory reaction than is seen in bronchopneumonia or lobar pneumonia. Mycoplasma pneumonia, caused by Mycoplasma pneumoniae, is referred to as primary atypical pneumonia and causes an inflammatory reaction similar to that of viral pneumonia.

Pneumonia can be caused by a variety of other fungal organisms, especially in debilitated persons such as those with cancer or AIDS. Mycobacterium tuberculosis, the causative agent of pulmonary tuberculosis, produces an inflammatory reaction similar to fungal organisms. See Mycobacterial diseases, Tuberculosis

Legionella pneumonia, initially called Legionnaire's disease, is caused by bacteria of the genus Legionella. The condition is frequently referred to under the broader name of legionellosis. See Legionnaires' disease

The signs and symptoms of pneumonia and pneumonitis are usually nonspecific, consisting of fever, chills, shortness of breath, and chest pain. Fever and chills are more frequently associated with infectious pneumonias but may also be seen in pneumonitis. The physical examination of a person with pneumonia or pneumonitis may reveal abnormal lung sounds indicative of regions of consolidation of lung tissue. A chest x-ray also shows the consolidation, which appears as an area of increased opacity (white area). Cultures of sputum or bronchial secretions may identify an infectious organism capable of causing the pneumonia.

The treatment of pneumonia and pneumonitis depends on the cause. Bacterial pneumonias are treated with antimicrobial agents. If the organisms can be cultured, the sensitivity of the organism to a specific antibiotic can be determined. Viral pneumonia is difficult to treat, as most drugs only help control the symptoms. The treatment of pneumonitis depends on identifying its cause; many cases are treated with cortisone-type medicines.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a group of diseases of the lungs characterized by inflammation of the alveolar, interstitial, and connective tissues of the lungs and of the bronchioles. The inflammation often spreads to the vascular system of the lungs as well. Pneumonia may be caused by viruses or bacteria or it may develop from physicochemical lesions resulting from burns, chemical substances, or war gases. The more common viral pneumonias are the influenzal, adenoviral, and ornithotic. The more common bacterial pneumonias are the pneumococcal, streptococcal, and staphylococcal, though the bacterial flora is usually mixed. The course of the disease may be acute or chronic. Acute pneumonia is one of the commonest respiratory diseases.

Bacteria and viruses penetrate into the lungs through the respiratory tract and, much less frequently, through the lymphatic and blood vessels. The development of pneumonia is also dependent on the so-called reactivity or resistance of the body. Low resistance may result from excessive fatigue, a preexisting disease, chilling, and harmful habits, such as the use of alcohol. Pneumonia may be lobar, lobular, or interstitial according to the nature of the changes in lung tissue.

Lobar pneumonia involves the lobe of a lung or a large portion of it and is characterized by a cyclical course and structural changes in the lungs. The pneumococcus frequently plays a part in the genesis of the inflammatory process, although other microflora may be significant. In typical cases, disease sets in suddenly, frequently with chills. Other symptoms include a rapid rise in body temperature to 39° C or higher, general weakness, headache, and cough, first dry and then with rust-colored sputum. An examination will indicate changes in the lungs and frequently in the pleura. Intoxication is clearly revealed, as well as impairment of the function of the cardiovascular and nervous systems, of the blood (leukocytosis), of metabolic processes, and of the kidneys. The duration of the disease is between two and three weeks. If antibiotic treatment is started early, the intoxication is corrected, the temperature falls, and the patient feels better from the third to the fifth day and sometimes even sooner. Complete recovery comes at about the same time.

Lobular pneumonias may be caused by a variety of agents and may vary considerably in their development and course. In contrast to lobar pneumonia, the inflammatory reaction in lobular pneumonia covers not a lobe but separate parts (lobules or groups of lobules) and takes the form of small and usually multiple foci. Since the disease frequently begins with affection of the bronchi, this form is also called bronchopneumonia. The symptoms of lobular pneumonia are highly varied, since they depend to a large extent on the causative agent and general condition of the patient. For example, pneumonia caused by the staphylococcus has a protracted course, frequently with no pronounced symptoms, and shows little response to antibiotic therapy. A general malaise and a less acute onset than in lobar pneumonia are common to all types of lobular pneumonia; headache, cough with mucopurulent sputum, and a rise in temperature to 37°-39° C may also occur. Examination reveals changes in lung tissue, a blunting of percussion resonance, and moist sonorous rales over some portions of the lungs. The causative agent can be found in the sputum. The duration of the disease is from two to three weeks.

Interstitial pneumonia is characterized by inflammatory reactions in the interstitial connective tissue of the lungs. Its symptoms are similar to those of lobular pneumonia. However, since it is impossible to obtain clear-cut data by examination, observation of the course of the disease and X-ray studies are of considerable diagnostic value.

Most cases of acute pneumonia respond to antibiotic treatment. But acute pneumonia may take a protracted course, become chronic, and occasionally be complicated by suppuration.

Chronic pneumonia is a protracted inflammation of the lungs lasting months or years in which not only lung (alveolar, interstitial) tissue but also the bronchioles and bronchi and the vascular and lymphatic systems are affected—that is, the structural elements of the lungs. Chronic pneumonia is characterized by intermittent exacerbations (flare-ups) followed by temporary remissions. The frequency and intensity of the exacerbations increase as the disease progresses. Chronic pneumonia is very often caused by acute pneumonia treated late or incorrectly, by a protracted course of acute pneumonia when resistance is low, and by the presence of chronic inflammatory processes in the upper respiratory tract and bronchi. Dusty air and smoking, among other factors, contribute to the disease. The symptoms of chronic pneumonia are quite varied, depending on the lung and bronchial structures involved in the process.

Preventive measures include observance of healthful and hygienic practices at work and in everyday life, control of air pollution and harmful habits such as smoking and the use of alcohol, and treatment of diseases of the upper respiratory tract. Chronic pneumonia can also be prevented by careful treatment of acute pneumonia. Treatment of acute pneumonia and exacerbations of chronic pneumonias should be carried out in hospitals as much as possible. Bed rest and a high-calorie diet enriched with vitamins are essential. Antibiotics or sulfanilamides, expectorants, inhalation of oxygen, cupping glasses, and mustard plasters are used. During remissions of chronic pneumonia steps are taken to increase general resistance: a conservative regimen, exercise, physical therapy procedures, administration of drugs to relieve bronchospasm, and sanatorium and health resort treatment on the Southern Crimean Shore, in mountain health resorts, or in localities with coniferous forests. Surgery is sometimes indicated for segmental injuries.


Molchanov, N. S. Ostrye pnevmonii. Leningrad, 1965.
Medvedev, V. V. “Khronicheskie nespetsificheskie pnevmonii.” In Mnogotomnoe rukovodstvo po vnutrennim bolezniam, vol. 3. Moscow, 1964. Pages 226-46.
Sil’vestrov, V. P. Zatianuvshiesia pnevmonii i ikh lechenie. Leningrad, 1968.


Pneumonia in animals In animals pneumonia occurs (generally in young animals) in the form of bronchopneumonia or lobar pneumonia. It follows excessive chilling, inhalation of irritants, and infectious and parasitic diseases, such as contagious pleuropneumonia of horses, pasteurellosis, and dictyocauliasis. Some of the symptoms are cough, fever, and dyspnea. Pneumonia in animals can be complicated by suppurative inflammation or gangrene of the lungs. Treatment and prevention are aimed at removing the causes; antibiotics, sulfanilamides, and other drugs are used.


Vnutrennie nezaraznye bolezni sel’skokhoziaistvennykh zhivotnykh, 3rd ed. Moscow, 1967.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


An acute or chronic inflammation of the lungs caused by numerous microbial, immunological, physical, or chemical agents, and associated with exudate in the alveolar lumens.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


inflammation of one or both lungs, in which the air sacs (alveoli) become filled with liquid, which renders them useless for breathing. It is usually caused by bacterial (esp pneumococcal) or viral infection
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005