Pneumonia is an acute infection of lung parenchyma. It is a heterogeneous entity defined on the basis of the situation in which it is diagnosed, the etiological organism and the host-co-morbidity. Pneumonias are commonly classified as community acquired (typical or atypical), nosocomial (hospital acquired, ventilator associated; normal host, immuno-compromised host and other co-morbidities), aspiration pneumonia, pneumonia in immuno-compromised host and pneumonias in the elderly.

Community-acquired pneumonia (CAP) refers to an infectious process resulting from the invasion and overgrowth of microorganisms in lung parenchyma in a non-hospitalized population i.e. development while living ‘in the community’.  On the contrary, pneumonia occurring 48 hours or more after admission in a hospital, is referred to as hospital-acquired pneumonia (HAP).  Pneumonia arising more than 48-72 hours after endotracheal intubation in an intensive care unit is called as ventilator-associated pneumonia (VAP).

Occurrence of fever and respiratory symptoms or signs (Cough, sputum production, blood in the sputum) are important symptoms. Chest pain is another common complaint. Patient may also complain of severe chills, headache, malaise and vomiting. The presence of a chest infiltrates on X-Ray are essential for the diagnosis of CAP.  Lung infiltrates are sometimes not evident on frontal radiography but computed tomography which may reveal such occult infiltrates is seldom required for routine diagnosis.

CAP may be classified by morphological patterns, etiological organism and empirical ‘host-organism pattern’ approach.  Etiologically, the most common bacterial causes of CAP include Streptococcus pneumoniae, and H. influenza.   Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella pneumophila are classified as atypical pneumonias.  Enteric gram-negatives, Staphylococcus aureus, anaerobes, Ps. Aeruginosa, viruses, mycobacteria and fungi are unusual causes of CAP and constitute less than 5% of cases together.

CAP is also defined on the basis of its response to therapy.  Some of these terms which are commonly used but imprecisely defined include ‘progressive pneumonia’, ‘slowly-resolving’ or ‘non-responding pneumonia’ and ‘non-resolving pneumonia’.  With appropriate antibiotics, resolution of symptoms begins at 48-72 hours.  The lack of complete radiological resolution at 12 weeks is described as ‘non-resolving pneumonia’.

CAP is an important cause of morbidity and mortality world-wide.  In the United States, pneumonia is the sixth leading cause of death.  The disease affects all age groups but particularly the children under the age of 5 years, the elderly and those with an underlying risk factor.  The annual incidence in those aged above 65 years is estimated between 25 and 55 cases per 1000 while the overall annual incidence of CAP is around  ~2%.  In the U.S., the overall attack rate is about 12 cases per 1000 persons per year.  The incidence is also high in people living in old-age homes or other similar institutions.

The prevalence is reported as high in the developing countries.  Of 15 million deaths in children from acute respiratory infections worldwide, about a third result from pneumonia.  This is attributable to both the environmental and the host risk factors.  There are limited data on the epidemiology and population prevalence of CAP in India.    Whatever data is reported is drawn from the hospitalized patients.  A few other hospital based studies are also reported on clinical and/or microbiological features of atypical forms of CAPs, such as the Mycoplasma and Chlamydia pneumonia especially in children.   Mycoplasma pneumonia was reported in 27.4%.  Of 62 children admitted with CAP in a one year period in Delhi – only 4 (6.4%) were reported as seropositive for chlamydia.

Streptococcus pneumonia remains the most common cause of CAP in India as anywhere else in the world.  In a multicentre hospital surveillance for Strep pneumonia, in six hospitals in India over a period of 4 years, there were 3686 cases with suspected pneumonia and the clinical syndrome of pneumonia was identified in 93 out of 314 cases (29.6%) with invasive pneumococcal disease.  A 19% case fatality of pneumonia was also described.

The pathogenesis of CAP involves an imbalance between the pathogen’s invasiveness and the host immunity.  What Louis Pasteur had said more than a hundred years ago about the host factors, is revealing – ‘The germ is nothing; the soil is everything’.  In general, there are numerous host factors which predispose to CAP.  While the virulence of the microorganism depending upon the group, the strain and the type (etc.) is the root cause of infection, the host factors determine the occurrence and the severity.  Infant, young children and the elderly are the most vulnerable.  There are several predisposing and disease modifying factors which include the presence of a systemic illness, a behavioral problem or an extraneous environmental exposure.

Some risk factors are also identifiable for specific types of CAPs caused by different microorganisms.  For example specific risk factors for pneumococcal infections include dementia, seizures, HIV infection and systemic illnesses such as the chronic obstructive pulmonary disease (COPD), congestive heart failure (CHF) and cerebrovascular disease.  Haemophilus influenzae is most common in patients with an underlying COPD and the use of antibiotics or oral corticosteroids within the past 3 months.  The risk factors for some of the relatively uncommon CAPs such as those caused by pseudomonas, Staph aureus, gram negative bacteria, drug resistant streptococcus pneumoniae (DRSP) or anaerobic organisms are generally identifiable.

Immunosuppression with drugs or HIV infection is an important risk factor on the rise in the recent years.  Strep pneumoniae and H. influenzae are the common organisms in the developed as well as the developing world.   Such patients are also likely to develop CAPs with different atypical organisms including the mycobacteria, the viruses (such as the cytomegalovirus), herpes simplex, influenza virus and respiratory syncytial virus) and the fungi.

Tuberculosis is another important respiratory infection in HIV positive patients which may present with pulmonary infiltrates and clinical features of CAP28-30.  Tuberculosis therefore requires to be kept in the differential diagnosis of CAP in case of all immuno-suppressed patients especially those with HIV infection.

The diagnosis is often made on clinical ground.   Pneumonia diagnosis may also

require the need for chest CT scanning to clearly visualize the lung fields. Sputum and

sometimes, fiberoptic bronchoscopy may be required to obtain bronchial secretions for

examination for the responsible micro-organisms. Culture sensitivity tests are done to

look for the drugs to which the organisms are sensitive.

Most of the times, treatment is given on empirical grounds since the organisms may

not always be known. In particular, CAP can be successfully managed without any

culture-sensitivity report. There are standard management guidelines to choose the

most appropriate antibiotic for a particular patient in a particular situation.