Pneumonia is an inflammatory condition of the lung affecting primarily the microscopic air sacs known as alveoli.

It is usually caused by infection with viruses or bacteria and less commonly other microorganisms, certain drugs and other conditions such as autoimmune diseases.

Typical symptoms include a cough, chest pain, fever, and difficulty breathing.

Diagnostic tools include x-rays and the culture of the sputum. Vaccines to prevent certain types of pneumonia are available.

Treatment depends on the underlying cause. Pneumonia presumed to be bacterial is treated with antibiotics.

If the pneumonia is severe, the affected person is admitted to the hospital.

Pneumonia affects approximately 450 million people globally per year, seven percent of the population, and results in about 4 million deaths, mostly in developing countries.

In developing countries, and among the very old, the very young, and the chronically ill, pneumonia remains a leading cause of death.

In the terminally ill and elderly, especially those with other conditions, pneumonia is often the immediate cause of death.

In such cases, particularly when it cuts short the suffering associated with a lingering illness, pneumonia has often been called “the old man’s friend.”

People with infectious pneumonia often have a productive cough, fever accompanied by shaking chills, shortness of breath, sharp or stabbing chest pain during deep breaths, and an increased respiratory rate.

In the elderly, confusion may be the most prominent sign.

The typical signs and symptoms in children under five are fever, cough, and fast or difficult breathing.

Fever is not very specific, as it occurs in many other common illnesses, and maybe absent in those with severe disease or malnutrition.

In addition, a cough is frequently absent in children less than 2 months old.

More severe signs and symptoms may include blue-tinged skin, decreased thirst, convulsions, persistent vomiting, extremes of temperature, or a decreased level of consciousness.

Bacterial and viral cases of pneumonia usually present with similar symptoms.

Some causes are associated with classic, but non-specific, clinical characteristics.

Pneumonia caused by Legionella may occur with abdominal pain, diarrhea, or confusion, while pneumonia caused by Streptococcus pneumoniae is associated with rusty colored sputum, and pneumonia caused by Klebsiella may have bloody sputum often described as “currant jelly”.

Bloody sputum (known as hemoptysis) may also occur with tuberculosis, Gram-negative pneumonia, and lung abscesses as well as more commonly with acute bronchitis.

Mycoplasma pneumonia may occur in association with swelling of the lymph nodes in the neck, joint pain, or a middle ear infection.

Viral pneumonia presents more commonly with wheezing than does bacterial pneumonia.

Pneumonia is due to infections caused primarily by bacteria or viruses and less commonly by fungi and parasites.

Although there are more than 100 strains of infectious agents identified, only a few are responsible for the majority of the cases.

Mixed infections with both viruses and bacteria may occur in up to 45% of infections in children and 15% of infections in adults.

A causative agent may not be isolated in approximately half of the cases despite careful testing.

The term pneumonia is sometimes more broadly applied to any condition resulting in inflammation of the lungs (caused for example by autoimmune diseases, chemical burns or drug reactions); however, this inflammation is more accurately referred to as pneumonitis.

Infective agents were historically divided into “typical” and “atypical” based on their presumed presentations, but the evidence has not supported this distinction, thus it is no longer emphasized.

Conditions and risk factors that predispose to pneumonia include smoking, immunodeficiency, alcoholism, chronic obstructive pulmonary disease, chronic kidney disease, and liver disease.

The use of acid-suppressing medications—such as proton-pump inhibitors or H2 blockers—is associated with an increased risk of pneumonia. The risk is also increased in old age.

Bacteria are the most common cause of community-acquired pneumonia (CAP), with Streptococcus pneumonia isolated in nearly 50% of cases.

A number of drug-resistant versions of the above infections are becoming more common, including drug-resistant Streptococcus pneumoniae (DRSP) and methicillin-resistant Staphylococcus aureus (MRSA).

The spreading of organisms is facilitated when risk factors are present.

Streptococcus pneumonia is more common in the winter.

Pneumonia frequently starts as an upper respiratory tract infection that moves into the lower respiratory tract.

Viruses may reach the lung by a number of different routes.

The respiratory syncytial virus is typically contracted when people touch contaminated objects and then they touch their eyes or nose. Other viral infections occur when contaminated airborne droplets are inhaled through the mouth or nose.

Once in the upper airway, the viruses may make their way in the lungs, where they invade the cells lining the airways, alveoli, or lung parenchyma.

Some viruses such as measles and herpes simplex may reach the lungs via the blood. The invasion of the lungs may lead to varying degrees of cell death.

When the immune system responds to the infection, more lung damage may occur. Primarily White blood cells, mainly mononuclear cells, generate the inflammation.

As well as damaging the lungs, many viruses simultaneously affect other organs and thus disrupt other body functions.

Viruses also make the body more susceptible to bacterial infections; in this way, bacterial pneumonia can arise as a co-morbid condition (pertaining to a disease or other pathological process that occurs simultaneously with another.)

Diagnosis

Pneumonia is typically diagnosed based on a combination of physical signs and a chest X-ray.

However, the underlying cause can be difficult to confirm, as there is no definitive test able to distinguish between bacterial and non-bacterial origin.

There is a very low risk of pneumonia if all vital signs and auscultation are normal.

In persons requiring hospitalization, pulse oximetry, chest radiography and blood tests—including a complete blood count, serum electrolytes, C-reactive protein level, and possibly liver function tests—are recommended.

The diagnosis of influenza-like illness can be made based on the signs and symptoms; however, confirmation of an influenza infection requires testing. Thus, treatment is frequently based on the presence of influenza in the community or a rapid influenza test.

Physical exam

Physical examination may sometimes reveal low blood pressure, high heart rate, or low oxygen saturation.

The respiratory rate may be faster than normal, and this may occur a day or two before other signs.

Examination of the chest may be normal but may show decreased chest expansion on the affected side.

Harsh breath sounds from the larger airways that are transmitted through the inflamed lung are termed bronchial breathing and are heard on auscultation with a stethoscope.

Crackles (rales) may be heard over the affected area during inspiration. Percussion may be dulled over the affected lung, and increased, rather than decreased, vocal resonance distinguishes pneumonia from a pleural effusion.

Imaging

A chest radiograph is frequently used in diagnosis. In people with mild disease, imaging is needed only in those with potential complications, those not having improved with treatment or those in which the cause is uncertain.

If a person is sufficiently sick to require hospitalization, a chest radiograph is recommended.

Findings do not always match the severity of disease and do not reliably separate between bacterial infection and viral infection.

X-ray presentations of pneumonia may be classified as lobar pneumonia, bronchopneumonia (also known as lobular pneumonia), and interstitial pneumonia.

Bacterial, community-acquired pneumonia classically shows lung consolidation of one lung segmental lobe, which is known as lobar pneumonia.

However, findings may vary, and other patterns are common in other types of pneumonia. Aspiration pneumonia may present with bilateral opacities primarily in the bases of the lungs and on the right side.

Radiographs of viral pneumonia may appear normal, appear hyper-inflated, have bilateral patchy areas, or present similar to bacterial pneumonia with lobar consolidation.

Radiologic findings may not be present in the early stages of the disease, especially in the presence of dehydration, or may be difficult to be interpreted in the obese or those with a history of lung disease. A CT scan can give additional information in indeterminate cases.

Prognosis

With treatment, most types of bacterial pneumonia will stabilize in 3–6 days. It often takes a few weeks before most symptoms resolve.

X-ray finding typically clear within four weeks and mortality is low (less than 1%).

In the elderly or people with other lung problems, recovery may take more than 12 weeks. In persons requiring hospitalization, mortality may be as high as 10%, and in those requiring intensive care, it may reach 30–50%.

Pneumonia is the most common hospital-acquired infection that causes death.

Complications may occur in particular in the elderly and those with underlying health problems.

This may include, among others: empyema, lung abscess, bronchiolitis obliterans, acute respiratory distress syndrome, sepsis, and worsening of underlying health problems.

Respiratory and circulatory failure

Pneumonia can cause respiratory failure by triggering acute respiratory distress syndrome (ARDS), which results from a combination of infection and inflammatory response.

The lungs quickly fill with fluid and become stiff. This stiffness, combined with severe difficulties extracting oxygen due to the alveolar fluid, may require long periods of mechanical ventilation for survival.

Sepsis is a potential complication of pneumonia.

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