The incidence of pleural empyema and the prevalence of specific causative microorganisms varies depending on the source of infection (community acquired vs. hospital acquired pneumonia), the age of the patient and host immune status. Risk factors include alcoholism, drug use, HIV infection, neoplasm and pre-existent pulmonary disease. Pleural empyema was found in 0.7% of 3675 patients needing hospitalization for a community acquired pneumonia in a recent Canadian single-center prospective study. A multi-center study from the UK including 430 adult patients with community acquired pleural empyema found negative pleural-fluid cultures in 54% of patients, Streptococcus milleri group in 16%, Staphylococcus aureus in 12%, Streptococcus pneumoniae in 8%, other Streptococci in 7% and anaerobic bacteria in 8%. Given the difficulties in culturing anaerobic bacteria the frequency of the latter (including mixed infections) might be underestimated.

The risk of empyema in children seems to be comparable to adults. Using the United States Kids’ Inpatient Database the incidence is calculated to be around 1.5% in children hospitalized for community acquired pneumonia, although percentages up to 30% have been reported in individual hospitals, a difference which may be explained by an transient endemic of highly invasive serotype or overdiagnosis of small parapneumonic effusions. The distribution of causative organisms does differ greatly from that in adults: in an analysis of 78 children with community acquired pleural empyema, no micro-organism was found in 27% of patients, Streptococcus pneumoniae in 51%, Streptococcus pyogenes in 9% and Staphylococcus aureus in 8%.

Although pneumococcal vaccination dramatically decreased the incidence of pneumonia in children, it did not have this effect on the incidence of complicated pneumonia. It has been shown that the incidence of empyema in children was already on the rise at the end of the 20th century, and that the widespread use of pneumococcal vaccination did not slow down this trend. This might in part be explained by a change in prevalence of (more invasive) pneumococcal serotypes, some of which are not covered by the vaccine, as well a rise in incidence of pneumonia caused by other streptococci and staphylococci. The incidence of empyema seems to be rising in the adult population as well, albeit at a slower rate.

All patients with empyema require outpatient follow-up with a repeat chest X-ray and inflammatory biochemistry analysis within 4 weeks following discharge. Chest radiograph returns to normal in the majority of patients by 6 months. Patients should of course be advised to return sooner if symptoms redevelop. Long-term sequelae of pleural empyema are rare but include bronchopleural fistula formation, recurrent empyema and pleural thickening, which may lead to functional lung impairment needing surgical decortication.

Approximately 15% of adult patients with pleural infection die within 1 year of the event, although deaths are usually due to comorbid conditions and not directly due to sepsis from the empyema. Mortality in children is generally reported to be less than 3%. No reliable clinical, radiological or pleural fluid characteristics accurately determine patients’ prognosis at initial presentation.

If left untreated, miliary tuberculosis is almost always fatal. Although most cases of miliary tuberculosis are treatable, the mortality rate among children with miliary tuberculosis remains 15 to 20% and for adults 25 to 30%. One of the main causes for these high mortality rates includes late detection of disease caused by non-specific symptoms. Non-specific symptoms include: coughing, weight loss, or organ dysfunction. These symptoms may be implicated in numerous disorders, thus delaying diagnosis. Misdiagnosis with tuberculosis meningitis is also a common occurrence when patients are tested for tuberculosis, since the two forms of tuberculosis have high rates of co-occurrence.

When a pleural effusion has been determined to be exudative, additional evaluation is needed to determine its cause, and amylase, glucose, pH and cell counts should be measured.

- Red blood cell counts are elevated in cases of bloody effusions (for example after heart surgery or hemothorax from incomplete evacuation of blood).

- Amylase levels are elevated in cases of esophageal rupture, pancreatic pleural effusion, or cancer.

- Glucose is decreased with cancer, bacterial infections, or rheumatoid pleuritis.

- pH is low in empyema (<7.2) and may be low in cancer.

- If cancer is suspected, the pleural fluid is sent for cytology. If cytology is negative, and cancer is still suspected, either a thoracoscopy, or needle biopsy of the pleura may be performed.

- Gram staining and culture should also be done.

- If tuberculosis is possible, examination for "Mycobacterium tuberculosis" (either a Ziehl–Neelsen or Kinyoun stain, and mycobacterial cultures) should be done. A polymerase chain reaction for tuberculous DNA may be done, or adenosine deaminase or interferon gamma levels may also be checked.

The most common causes of exudative pleural effusions are bacterial pneumonia, cancer (with lung cancer, breast cancer, and lymphoma causing approximately 75% of all malignant pleural effusions), viral infection, and pulmonary embolism.

Another common cause is after heart surgery, when incompletely drained blood can lead to an inflammatory response that causes exudative pleural fluid.

Conditions associated with exudative pleural effusions:

- Parapneumonic effusion due to pneumonia

- Malignancy (either lung cancer or metastases to the pleura from elsewhere)

- Infection (empyema due to bacterial pneumonia)

- Trauma

- Pulmonary infarction

- Pulmonary embolism

- Autoimmune disorders

- Pancreatitis

- Ruptured esophagus (Boerhaave's syndrome)

- Rheumatoid pleurisy

- Drug-induced lupus

CAP is common worldwide, and a major cause of death in all age groups. In children, most deaths (over two million a year) occur in newborn period. According to a World Health Organization estimate, one in three newborn deaths are from pneumonia. Mortality decreases with age until late adulthood, with the elderly at risk for CAP and its associated mortality.

More CAP cases occur during the winter than at other times of the year. CAP is more common in males than females, and more common in black people than Caucasians. Patients with underlying illnesses (such as Alzheimer's disease, cystic fibrosis, COPD, tobacco smoking, alcoholism or immune-system problems) have an increased risk of developing pneumonia.

Major complications of CAP include:

- Sepsis, when microorganisms enter the bloodstream and the immune system responds. Sepsis often occurs with bacterial pneumonia, with "streptococcus pneumoniae" the most-common cause. Patients with sepsis require intensive care, with blood-pressure monitoring and support against hypotension. Sepsis can cause liver, kidney and heart damage.

- Respiratory failure: CAP patients often have dyspnea, which may require support. Non-invasive machines (such as bilevel positive airway pressure), a tracheal tube or a ventilator may be used.

- Pleural effusion and empyema: Microorganisms from the lung may trigger fluid collection in the pleural cavity. If the microorganisms are in the fluid, the collection is an empyema. If pleural fluid is present, it should be collected with a needle and examined. Depending on the results, complete drainage of the fluid with a chest tube may be necessary. If the fluid is not drained, bacteria may continue to proliferate because antibiotics do not penetrate the pleural cavity well.

- Abscess: A pocket of fluid and bacteria may be seen on an X-ray as a cavity in the lung. Abscesses, typical of aspiration pneumonia, usually contain a mixture of anaerobic bacteria. Although antibiotics can usually cure abscesses, sometimes they require drainage by a surgeon or radiologist.

HIV-infected children less than 12 years of age also develop disseminated MAC. Some age adjustment is necessary when clinicians interpret CD4+ T-lymphocyte counts in children less than 2 years of age. Diagnosis, therapy, and prophylaxis should follow recommendations similar to those for adolescents and adults.

An empyema (from Greek ἐμπύημα, "abscess") is a collection or gathering of pus within a naturally existing anatomical cavity. For example, pleural empyema is empyema of the pleural cavity. It must be differentiated from an abscess, which is a collection of pus in a newly formed cavity.

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. Before the advent of antibiotics, mortality was typically 30% in those that were hospitalized.

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.

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 but occurs usually in people with poor immunity or hyposplenism. The organisms most commonly involved are "Streptococcus pneumoniae", "Haemophilus influenzae", and "Klebsiella pneumoniae". Other causes of the symptoms should be considered such as a myocardial infarction or a pulmonary embolism.

Other causes of pleural effusion include tuberculosis (though stains of pleural fluid are only rarely positive for acid-fast bacilli, this is the most common cause of pleural effusions in some developing countries), autoimmune disease such as systemic lupus erythematosus, bleeding (often due to chest trauma), chylothorax (most commonly caused by trauma), and accidental infusion of fluids.

Less common causes include esophageal rupture or pancreatic disease, intra-abdominal abscesses, rheumatoid arthritis, asbestos pleural effusion, mesothelioma, Meigs' syndrome (ascites and pleural effusion due to a benign ovarian tumor), and ovarian hyperstimulation syndrome.

Pleural effusions may also occur through medical or surgical interventions, including the use of medications (pleural fluid is usually eosinophilic), coronary artery bypass surgery, abdominal surgery, endoscopic variceal sclerotherapy, radiation therapy, liver or lung transplantation, and intra- or extravascular insertion of central lines.

In human medicine, empyema occurs in:

- the pleural cavity (pleural empyema also known as pyothorax)

- the thoracic cavity

- the uterus (pyometra)

- the appendix (appendicitis)

- the meninges (subdural empyema)

- the joints (septic arthritis)

- the gallbladder

A parapneumonic effusion is a type of pleural effusion that arises as a result of a pneumonia, lung abscess, or bronchiectasis. There are three types of parapneumonic effusions: uncomplicated effusions, complicated effusions, and empyema. Uncomplicated effusions generally respond well to appropriate antibiotic treatment.

- Diagnosis

The criteria for a complicated parapneumonic effusion include the presence of pus, Gram stain–positive or culture-positive pleural fluid, pleural fluid pH <7.20, and pleural fluid LDH that is greater than three times the upper limit of normal of serum LDH. Diagnostic techniques available include plain film chest x-ray, computed tomography (CT), and ultrasound. Ultrasound can be useful in differentiating between empyema and other transudative and exudative effusions due in part to relative echogenicity of different organs such as the liver (often isoechogenic with empyema).

- Treatment

Appropriate management includes chest tube drainage (tube thoracostomy). Treatment of empyemas includes antibiotics, complete pleural fluid drainage, and reexpansion of the lung.

Other treatments include the use of decortication.

Subdural empyaema is a form of empyema in the subdural space.

Bacterial or occasionally fungal infection of the skull bones or air sinuses can spread to the subdural space, producing a subdural empyema. The underlying arachnoid and subarachnoid spaces are usually unaffected, but a large subdural empyema may produce a mass effect. Further, a thrombophlebitis may develop in the bridging veins that cross the subdural space, resulting in venous occlusion and infarction of the brain. With treatment, including surgical drainage, resolution of the empyema occurs from the dural side, and, if it is complete, a thickened dura may be the only residual finding. Symptoms include those referable to the source of the infection. In addition, most patients are febrile, with headache and neck stiffness, and, if untreated, may develop focal neurologic signs, lethargy, and coma. The CSF profile is similar to that seen in brain abscesses, because both are parameningeal infectious processes. If diagnosis and treatment are prompt, complete recovery is usual.

It usually occurs in infancy. It can be associated with sinusitis.

Chronic mediastinitis is usually a radiologic diagnosis manifested by diffuse fibrosis of the soft tissues of the mediastinum. This is sometimes the consequence of prior granulomatous disease, most commonly histoplasmosis. Other identifiable causes include tuberculosis, IgG4-related disease and radiation therapy. Fibrosing mediastinitis most frequently causes problems by constricting blood vessels or airways in the mediastinum. This may result in such complications as superior vena cava syndrome or pulmonary edema from compression of pulmonary veins.

Treatment for chronic fibrosing mediastinitis is somewhat controversial, and may include steroids or surgical decompression of affected vessels.

In the post-antibiotic era pattern of frequency is changing. In older studies anaerobes were found in up to 90% cases but they are much less frequent now.

The pleural space can be invaded by fluid, air, and particles from different parts of the body which fairly complicates the diagnosis. Viral infection (coxsackie B virus, HRSV, CMV, adenovirus, EBV, parainfluenza, influenza) is the most common cause of pleurisy. However, many other different conditions can cause pleuritic chest pain:

- Aortic dissections

- Autoimmune disorders such as systemic lupus erythematosus (or drug-induced lupus erythematosus), Autoimmune hepatitis (AIH), rheumatoid arthritis and Behçet's disease.

- Bacterial infections associated with pneumonia and tuberculosis

- Chest injuries (blunt or penetrating)

- Familial Mediterranean fever, an inherited condition that often causes fever and swelling in the abdomen or the lungs

- Fungal or parasitic infections

- Heart surgery, especially coronary-artery bypass grafting

- Cardiac problems (ischemia, pericarditis)

- Inflammatory bowel disease

- Lung cancer and lymphoma

- Other lung diseases like cystic fibrosis, sarcoidosis, asbestosis, lymphangioleiomyomatosis, and mesothelioma

- Pneumothorax

- Pulmonary embolisms, which are blood clots that enter the lungs

When the space between the pleurae starts to fill with fluid, as in pleural effusion, the chest pain can be eased but a shortness of breath can result, since the lungs need room to expand during breathing. Some cases of pleuritic chest pain are idiopathic, which means that the exact cause cannot be determined.

Tuberculous pericarditis is a form of pericarditis.

Pericarditis caused by tuberculosis is difficult to diagnose, because definitive diagnosis requires culturing "Mycobacterium tuberculosis" from aspirated pericardial fluid or pericardial , which requires high technical skill and is often not diagnostic (the yield from culture is low even with optimum specimens). The Tygerberg scoring system helps the clinician to decide whether pericarditis is due to tuberculosis or whether it is due to another cause: night sweats (1 point), weight loss (1 point), fever (2 point), serum globulin > 40g/l (3 points), blood total leucocyte count <10 x 10/l (3 points); a total score of 6 or more is highly suggestive of tuberculous pericarditis. Pericardial fluid with an interferon-γ level greater than 50/ml is highly specific for tuberculous pericarditis.

There are no randomized trials which evaluate the length of anti-tuberculosis treatment required for tuberculous pericarditis. There is a small but not conclusive benefit for treatment with a schedule of steroids with anti-tuberculosis drugs. Open surgical drainage of fluid though effective in preventing cardiac tamponade was associated with more deaths.

Most cases respond to antibiotics and prognosis is usually excellent unless there is a debilitating underlying condition. Mortality from lung abscess alone is around 5% and is improving.

Before the development of modern cardiovascular surgery, cases of acute mediastinitis usually arose from either perforation of the esophagus or from contiguous spread of odontogenic or retropharyngeal infections. However, in modern practice, most cases of acute mediastinitis result from complications of cardiovascular or endoscopic surgical procedures.

Treatment usually involves aggressive intravenous antibiotic therapy and hydration. If discrete fluid collections or grossly infected tissue have formed (such as abscesses), they may have to be surgically drained or debrided.

The standard treatment recommended by the WHO is with isoniazid and rifampicin for six months, as well as ethambutol and pyrazinamide for the first two months. If there is evidence of meningitis, then treatment is extended to twelve months. The U.S. guidelines recommend nine months' treatment. "Common medication side effects a patient may have such as inflammation of the liver if a patient is taking pyrazinamide, rifampin, and isoniazid. A patient may also have drug resistance to medication, relapse, respiratory failure, and adult respiratory distress syndrome."

MAI is common in immunocompromised individuals, including senior citizens and those with HIV/AIDS or cystic fibrosis. Bronchiectasis, the bronchial condition which causes unnatural enlargement of the bronchial tubes, is commonly found with MAI infection. Whether the bronchiectasis leads to the MAC infection or is the result of it is not always known.

The "Mycobacterium avium complex" (MAC) includes common atypical bacteria, i.e. nontuberculous mycobacteria (NTM), found in the environment which can infect people with HIV and low CD4 cell count (below 100/microliter); mode of infection is usually inhalation or ingestion.

MAC causes disseminated disease in up to 40% of people with human immunodeficiency virus (HIV) in the United States, producing fever, sweats, weight loss, and anemia. Disseminated MAC characteristically affects people with advanced HIV disease and peripheral CD4+ T-lymphocyte counts less than 100 cells/uL. Effective prevention and therapy of MAC has the potential to contribute substantially to improved quality of life and duration of survival for HIV-infected persons.

A number of factors make people more susceptible to TB infections. The most important risk factor globally is HIV; 13% of all people with TB are infected by the virus. This is a particular problem in sub-Saharan Africa, where rates of HIV are high. Of people without HIV who are infected with tuberculosis, about 5–10% develop active disease during their lifetimes; in contrast, 30% of those coinfected with HIV develop the active disease.

Tuberculosis is closely linked to both overcrowding and malnutrition, making it one of the principal diseases of poverty. Those at high risk thus include: people who inject illicit drugs, inhabitants and employees of locales where vulnerable people gather (e.g. prisons and homeless shelters), medically underprivileged and resource-poor communities, high-risk ethnic minorities, children in close contact with high-risk category patients, and health-care providers serving these patients.

Chronic lung disease is another significant risk factor. Silicosis increases the risk about 30-fold. Those who smoke cigarettes have nearly twice the risk of TB compared to nonsmokers.

Other disease states can also increase the risk of developing tuberculosis. These include alcoholism and diabetes mellitus (three-fold increase).

Certain medications, such as corticosteroids and infliximab (an anti-αTNF monoclonal antibody), are becoming increasingly important risk factors, especially in the developed world.

Genetic susceptibility also exists, for which the overall importance remains undefined.

Pleurisy and other disorders of the pleurae can be serious, depending on what caused them. Generally, pleurisy treatment has an excellent prognosis, but if left untreated it can cause severe complications. For example, a resulting pulmonary heart disease cor pulmonale, which manifests itself with an inflammation of the arms and legs, can lead to heart failure. If the conditions that caused the pleurisy or other pleural disorders were adequately diagnosed and treated early, one can expect a full recovery. Help of a pulmonologist (respiratory physician in the U.K. and Australia) may be enlisted to address the underlying cause and chart post-illness rehabilitation.

Progression from TB infection to overt TB disease occurs when the bacilli overcome the immune system defenses and begin to multiply. In primary TB disease (some 1–5% of cases), this occurs soon after the initial infection. However, in the majority of cases, a latent infection occurs with no obvious symptoms. These dormant bacilli produce active tuberculosis in 5–10% of these latent cases, often many years after infection.

The risk of reactivation increases with immunosuppression, such as that caused by infection with HIV. In people coinfected with "M. tuberculosis" and HIV, the risk of reactivation increases to 10% per year. Studies using DNA fingerprinting of "M. tuberculosis" strains have shown reinfection contributes more substantially to recurrent TB than previously thought, with estimates that it might account for more than 50% of reactivated cases in areas where TB is common. The chance of death from a case of tuberculosis is about 4% as of 2008, down from 8% in 1995.