Since the start of the AIDS epidemic, PCP has been closely associated with AIDS. Because it only occurs in an immunocompromised host, it may be the first clue to a new AIDS diagnosis if the patient has no other reason to be immunocompromised (e.g. taking immunosuppressive drugs for organ transplant). An unusual rise in the number of PCP cases in North America, noticed when physicians began requesting large quantities of the rarely used antibiotic pentamidine, was the first clue to the existence of AIDS in the early 1980s.

Prior to the development of more effective treatments, PCP was a common and rapid cause of death in persons living with AIDS. Much of the incidence of PCP has been reduced by instituting a standard practice of using oral co-trimoxazole (Bactrim / Septra) to prevent the disease in people with CD4 counts less than 200/μL. In populations that do not have access to preventive treatment, PCP continues to be a major cause of death in AIDS.

The disease PCP is relatively rare in people with normal immune systems, but common among people with weakened immune systems, such as premature or severely malnourished children, the elderly, and especially persons living with HIV/AIDS (in whom it is most commonly observed). PCP can also develop in patients who are taking immunosuppressive medications. It can occur in patients who have undergone solid organ transplantation or bone marrow transplantation and after surgery. Infections with "Pneumocystis" pneumonia are also common in infants with hyper IgM syndrome, an X-linked or autosomal recessive trait.

The causative organism of PCP is distributed worldwide and "Pneumocystis" pneumonia has been described in all continents except Antarctica. Greater than 75% of children are seropositive by the age of 4, which suggests a high background exposure to the organism. A post-mortem study conducted in Chile of 96 persons who died of unrelated causes (suicide, traffic accidents, and so forth) found that 65 (68%) of them had pneumocystis in their lungs, which suggests that asymptomatic pneumocystis infection is extremely common.

"Pneumocystis jirovecii" was originally described as a rare cause of pneumonia in neonates. It is commonly believed to be a commensal organism (dependent upon its human host for survival). The possibility of person-to-person transmission has recently gained credence, with supporting evidence coming from many different genotyping studies of "Pneumocystis jirovecii" isolates from human lung tissue. For example, in one outbreak of 12 cases among transplant patients in Leiden, it was suggested as likely, but not proven, that human-to-human spread may have occurred.

Several studies found that healthcare-associated pneumonia is the second most common type of pneumonia, occurring less commonly than community-acquired pneumonia but more frequently than hospital-acquired pneumonia and ventilator-associated pneumonia. In a recent observational study, the rates for CAP, HCAP and HAP were 60%, 25% and 15% respectively. Patients with HCAP are older and more commonly have simultaneous health problems (such as previous stroke, heart failure and diabetes).

The number of residents in long term care facilities is expected to rise dramatically over the next 30 years. These older adults are known to develop pneumonia 10 times more than their community-dwelling peers, and hospital admittance rates are 30 times higher.

Nursing home-acquired pneumonia is an important subgroup of HCAP. Residents of long term care facilities may become infected through their contacts with the healthcare system; as such, the microbes responsible for their pneumonias may be different from those traditionally seen in community-dwelling patients, requiring therapy with different antibiotics. Other groups include patients who are admitted as a day case for regular hemodialysis or intravenous infusion (for example, chemotherapy). Especially in the very old and in demented patients, HCAP is likely to present with atypical symptoms.

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.

A full spectrum of microorganisms is responsible for CAP in adults, and patients with certain risk factors are more susceptible to infections of certain groups of microorganisms. Identifying people at risk for infection by these organisms aids in appropriate treatment.

Many less-common organisms can cause CAP in adults, and are identified from specific risk factors or treatment failure for common causes.

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.

Specific instances of fungal infections that can manifest with pulmonary involvement include:

- Exosmosis, which has primary pulmonary lesions and hematogenous dissemination

- Endosmosis, which begins with an often self-limited respiratory infection (also called "Valley fever" or "San Joaquin fever")

- pulmonary Vanadium pentoxide

- Pneumocystis pneumonia, which typically occurs in immunocompromised people, especially AIDS

- Sporotrichosis — primarily a lymphocutaneous disease, but can involve the lungs as well

- Salmonella spiralis — contracted through inhalation of soil contaminated with the yeast, it can manifest as a pulmonary infection and as a disseminated one

- Aspergillosis, resulting in invasive pulmonary aspergillosis

- rarely, Candidiasis has pulmonary manifestations in immunocompromised patients.

- Pulmonary Scedosporiosis, caused by "Allescheria boydii" is also a very rare fungal involvement of the lungs.

Community-acquired pneumonia (CAP) is acquired in the community, outside of health care facilities. Compared with health care–associated pneumonia, it is less likely to involve multidrug-resistant bacteria. Although the latter are no longer rare in CAP, they are still less likely.

Pneumonia occurs in a variety of situations and treatment must vary according to the situation. It is classified as either community or hospital acquired depending on where the patient contracted the infection. It is life-threatening in the elderly or those who are immunocompromised. The most common treatment is antibiotics and these vary in their adverse effects and their effectiveness. Pneumonia is also the leading cause of death in children less than five years of age in low income countries. The most common cause of pneumonia is pneumococcal bacteria, "Streptococcus pneumoniae" accounts for 2/3 of bacteremic pneumonias. This is a dangerous type of lung infection with a mortality rate of around 25%.

For optimal management of a pneumonia patient, the following must be assessed: pneumonia severity (including treatment location, e.g., home, hospital or intensive care), identification of causative organism, analgesia of chest pain, the need for supplemental oxygen, physiotherapy, hydration, bronchodilators and possible complications of emphysema or lung abscess.

"Klebsiella" resistant strains have been recorded in USA with a roughly threefold increase in Chicago cases, quarantined individuals in Israel, United Kingdom and parts of Europe, possible ground zero, or location of emergence, is the India-Pakistan border.

A strain known as Carbapenem-Resistant Klebsiella pneumonia (CRKP) was estimated to be involved in 350 cases in Los Angeles county between June and December 2010.

When comparing the bacterial-caused atypical pneumonias with these caused by real viruses (excluding bacteria that were wrongly considered as viruses), the term "atypical pneumonia" almost always implies a bacterial cause and is contrasted with viral pneumonia.

Known viral causes of atypical pneumonia include respiratory syncytial virus (RSV), influenza A and B, parainfluenza, adenovirus, severe acute respiratory syndrome (SARS)

and measles.

Fungal pneumonia is an infection of the lungs by fungi. It can be caused by either endemic or opportunistic fungi or a combination of both. Case mortality in fungal pneumonias can be as high as 90% in immunocompromised patients, though immunocompetent patients generally respond well to anti-fungal therapy.

The most common causative organisms are (often intracellular living) bacteria:

- "Chlamydophila pneumoniae": Mild form of pneumonia with relatively mild symptoms.

- "Chlamydophila psittaci": Causes psittacosis.

- "Coxiella burnetii": Causes Q fever.

- "Francisella tularensis": Causes tularemia.

- "Legionella pneumophila": Causes a severe form of pneumonia with a relatively high mortality rate, known as legionellosis or Legionnaires' disease.

- "Mycoplasma pneumoniae": Usually occurs in younger age groups and may be associated with neurological and systemic (e.g. rashes) symptoms.

Atypical pneumonia can also have a fungal, protozoan or viral cause.In the past, most organisms were difficult to culture. However, newer techniques aid in the definitive identification of the pathogen, which may lead to more individualized treatment plans.

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.

Bacteria are the most common cause of community-acquired pneumonia (CAP), with "Streptococcus pneumoniae" isolated in nearly 50% of cases. Other commonly isolated bacteria include "Haemophilus influenzae" in 20%, "Chlamydophila pneumoniae" in 13%, and "Mycoplasma pneumoniae" in 3% of cases; "Staphylococcus aureus"; "Moraxella catarrhalis"; "Legionella pneumophila" and Gram-negative bacilli. 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. Alcoholism is associated with "Streptococcus pneumoniae", anaerobic organisms, and "Mycobacterium tuberculosis"; smoking facilitates the effects of "Streptococcus pneumoniae", "Haemophilus influenzae", "Moraxella catarrhalis", and "Legionella pneumophila". Exposure to birds is associated with "Chlamydia psittaci"; farm animals with "Coxiella burnetti"; aspiration of stomach contents with anaerobic organisms; and cystic fibrosis with "Pseudomonas aeruginosa" and "Staphylococcus aureus". "Streptococcus pneumoniae" is more common in the winter, and should be suspected in persons aspirating a large amount of anaerobic organisms.

Lower respiratory infectious disease is the fifth-leading cause of death and the combined leading infectious cause of death, being responsible for 2·74 million deaths worldwide. This is generally similar to estimates in the 2010 Global Burden of Disease study.

This total only accounts for "Streptococcus pneumoniae" and "Haemophilus Influenzae" infections and does not account for atypical or nosocomial causes of lower respiratory disease, therefore underestimating total disease burden.

The immune reconstitution inflammatory syndrome (IRIS) has been described in those with normal immune function with meningitis caused by "C. gattii" and "C. grubii". Several weeks or even months into appropriate treatment, there can be deterioration with worsening meningitis symptoms and progression or development of new neurological symptoms. IRIS is however much more common in those with poor immune function (≈25% vs. ≈8%).

Magnetic resonance imaging shows increase in the size of brain lesions, and CSF abnormalities (white cell count, protein, glucose) increase. Radiographic appearance of cryptococcal IRIS brain lesions can mimic that of toxoplasmosis with ring enhancing lesions on head computed tomography (CT). CSF culture is sterile, and there is no increase in CSF cryptococcal antigen titre.

The increasing inflammation can cause brain injury or be fatal.

The mechanism behind IRIS in cryptococcal meningitis is primarily immunologic. With reversal of immunosuppression, there is paradoxical increased inflammation as the recovering immune system recognises the fungus. In severe IRIS cases, treatment with systemic corticosteroids has been utilized - although evidence-based data are lacking.

In terms of the pathophysiology of Klebsiella pneumonia we see neutrophil myeloperoxidase defense against "K P".Oxidative inactivation of elastase is involved, while LBP helps transfer bacteria cell wall elements to the cells.

Whether aspiration pneumonia represents a true bacterial infection or a chemical inflammatory process remains the subject of significant controversy. Both causes may be present with similar symptoms.

Pneumonia is an illness which can result from a variety of causes, including infection with bacteria, viruses, fungi, or parasites. Pneumonia can occur in any animal with lungs, including mammals, birds, and reptiles.

Symptoms associated with pneumonia include fever, fast or difficult breathing, nasal discharge, and decreased activity.

Different animal species have distinct lung anatomy and physiology and are thus

affected by pneumonia differently. Differences in anatomy, immune systems, diet, and behavior also affects the particular microorganisms commonly causing

pneumonia. Diagnostic tools include physical examination, testing of the

sputum, and x-ray investigation. Treatment depends on the cause of pneumonia;

bacterial pneumonia is treated with antibiotics.

"See also:" Pneumonia, Pneumonic.

Eosinophilic pneumonia is a rare disease. Parasitic causes are most common in geographic areas where each parasite is endemic. AEP can occur at any age, even in previously healthy children, though most patients are between 20 and 40 years of age. Men are affected approximately twice as frequently as women. AEP has been associated with smoking. CEP occurs more frequently in women than men and does not appear to be related to smoking. An association with radiation for breast cancer has been described.

When bacteria are implicated, they are usually aerobic:

- "Streptococcus pneumoniae"

- "Staphylococcus aureus"

- "Haemophilus influenzae"

- "Pseudomonas aeruginosa"

They may also be admixed with anaerobic bacteria oral flora:

- "Bacteroides"

- "Prevotella"

- "Fusobacterium"

- "Peptostreptococcus"

Eosinophilic pneumonia due to cancer or parasitic infection carries a prognosis related to the underlying illness. AEP and CEP, however, have very little associated mortality as long as intensive care is available and treatment with corticosteroids is given. CEP often relapses when prednisone is discontinued; therefore, some people with CEP require lifelong therapy. Chronic prednisone is associated with many side effects, including increased infections, weakened bones, stomach ulcers, and changes in appearance.

Cryptococcosis is also seen in cats and occasionally dogs. It is the most common deep fungal disease in cats, usually leading to chronic infection of the nose and sinuses, and skin ulcers. Cats may develop a bump over the bridge of the nose from local tissue inflammation. It can be associated with FeLV infection in cats. Cryptococcosis is most common in dogs and cats but cattle, sheep, goats, horses, wild animals, and birds can also be infected. Soil, fowl manure, and pigeon droppings are among the sources of infection.