The word "pneumonia" takes its origin from the ancient Greek word "pneumon," which means "lung," so the word "pneumonia" becomes "lung disease." Medically it is an inflammation of one or both lungs' parenchyma that is more often, but not always, caused by infections. The many causes of pneumonia include bacteria, viruses, fungi, and parasites. This article will focus on bacterial pneumonia, as it is the major cause of morbidity and mortality. According to the new classification of pneumonia, there are four categories: community-acquired (CAP), hospital-acquired (HAP), healthcare-associated (HCAP) and ventilator-associated pneumonia (VAP). 
Types of Bacterial Pneumonia
Some articles include both HAP and VAP under the category of HCAP, so defining HCAP is problematic and controversial.
Community-acquired pneumonia can be caused by an extensive list of agents that include bacteria, viruses, fungi, and parasites, but this article will focus on bacterial pneumonia and its causes. Bacteria have classically been categorized into two divisions on the basis of etiology, "typical" and "atypical" organisms. Typical organisms can be cultured on standard media or seen on Gram stain, but "atypical" organisms do not have such properties. 
The most common cause of community-acquired pneumonia (CAP) is S. pneumoniae, followed by Klebsiella pneumoniae, Haemophilus influenzae, and Pseudomonas aeruginosa. The most common causes of HCAP and HAP are MRSA (methicillin-resistant Staphylococcus aureus) and Pseudomonas aeruginosa, respectively. The causative agents of VAP include both multi-drug resistant (MDR) agents (e.g., S. pneumoniae, other Strep spp, H. influenzae and MSSA) and non-MDR (e.g., P. aeruginosa, methicillin-resistant Staphylococcus aureus, Acinetobacter spp. and antibiotic-resistant Enterobacteriaceae) bacterial pathogens.
In the United States, lower respiratory tract infections account for more morbidity and mortality than any other infection.  The incidence of CAP in the United States is more than 5 million per year; 80% of these new cases are treated as outpatients with the mortality rate of less than 1%, and 20% are treated as inpatients with the mortality rate of 12% to 40%.
The incidence of CAP varies among different genders; for example, it is more common in males and African Americans than females and other Americans. However, the total number of deaths has been on the rise among females.  The incidence rates are higher at extremes of age; the adult rate is usually 5.15 to 7.06 cases per 1000 persons per year, but in the population of age less than 4 years and greater than 60 years, the rate is more than 12 cases per 1000 persons. In 2005, influenza and pneumonia combined was the eighth most common cause of death in the United States and the seventh most common cause of death in Canada. The mortality rate is variable among different regions, such as 7.3% for the United States and Canada, 9.1% for Europe, and 13.3% for Latin America.
The lower respiratory tract is not sterile, and it always is exposed to environmental pathogens. Invasion and propagation of the above-mentioned bacteria into lung parenchyma at alveolar level causes bacterial pneumonia, and the body's inflammatory response against it causes the clinical syndrome of pneumonia.
To prevent this proliferation of microorganisms there are a number of host defenses working together in lungs such as mechanical (e.g., hair in nostrils and mucus on nasopharynx and oropharynx) and chemical (e.g., proteins produced by alveolar epithelial cells like surfactant protein A and D, which have the intrinsic property of opsonizing bacteria). Another component of the pulmonary defense system is made up of immune cells such as alveolar macrophages, which work to engulf and kill proliferating bacteria, but once bacteria overcome the capacity of host defenses, they start proliferating. In this setting, the alveolar macrophages kickoff the inflammatory response to strengthen the lower respiratory tract defenses. This inflammatory response is the main reason for the clinical manifestation of bacterial pneumonia. Cytokines are released in response to the inflammatory reaction and cause the constitutional symptoms, for example, IL-1 (interleukin-1) and TNF (tumor necrosis factor) cause fever. Chemokine-like IL-8 (interleukin-8) and colony-stimulating factors like G-CSF (granulocyte colony-stimulating factor) promote chemotaxis and neutrophils maturation respectively, resulting in leukocytosis on serological lab and purulent secretions. These cytokines are responsible for the leakage of the alveolar-capillary membrane at the site of inflammation, causing a decrease in compliance and shortness of breath. Sometimes even erythrocytes cross this barrier and result in hemoptysis.
Pathologically, lobar pneumonia is the acute exudative inflammation of a lung lobe. It has the following four advanced stages if left untreated:
While taking the history, it is crucial to explore the patient's potential exposures, risks of aspiration, host factors, and presenting symptoms.
Exposure: A detailed history of possible exposures should be sought as it can help in establishing the potential etiologies. The following are some associations of exposures and etiologies of bacterial pneumonia:
Risks of Aspiration: Patients who have an increased risk of aspiration are more prone to develop pneumonia secondary to aspiration. Associated risks are:
Host mechanisms: It is of utmost importance to explore a detailed history to find clues towards the etiology of pneumonia. For instance, a history of asthma, COPD, smoking, and immunocompromised status can be indicative of H influenzae infection. H influenza most commonly appears in the winter season. Similarly, social, sexual, medication and family history can all be useful in determining the cause of illness.
Features in the history of bacterial pneumonia may vary from indolent to fulminant. Clinical manifestation includes both constitutional findings and findings due to damage to the lung and related tissue. The following are significant history findings:
For unbeknownst reasons, the presence of rigors is more often indicative of pneumococcal pneumonia than other bacterial pathogens. 
The presence of productive cough is the most common and significant presenting symptom. Some bacterial causes have particular manifestation, such as:
Atypical pneumonia presents with pulmonary and extra-pulmonary manifestations, such as Legionella pneumonia, often presents with altered mentation and gastrointestinal symptoms.
Physical findings also vary from patient to patient and mainly depend on the severity of lung consolidation, the type of organism, the extent of the infection, host factors, and existence or nonexistence of pleural effusion. The following are major clinical findings:
Confusion manifests earlier in older patients. A critically ill patient may present with sepsis or multi-organ failure.
Some examination findings are specific for certain etiologies, such as:
The approach to evaluate and diagnose pneumonia depends on the clinical status, laboratory parameters, and radiological evaluation.
In all patients with bacterial pneumonia, empirical therapy should be started as soon as possible. The first step in treatment is a risk assessment to know whether the patient should be treated in an outpatient or inpatient setting. Cardiopulmonary conditions, age, and severity of symptoms affect risk for bacterial pneumonia, especially CAP.
An expanded CURB-65 or CURB-65 pneumonia severity score can be used for risk quantification. It includes C = Confusion, U = Uremia (BUN greater than 20 mg/dL), R = Respiratory rate (greater than 30 per min), B = B.P (BP less than 90/60 mmHg) and age greater than 65 years. One point is scored for each of these risk factors. For a score of 0-1, outpatient treatment is advised. If the total score is 2 or more, it indicates medical ward admission. If the total score is 3 or more, it indicates ICU admission. Recommended therapy for different settings are as follows:
After getting a culture-positive lab result, therapy should be altered according to the culture-specific pathogen.
The patient also can benefit from smoking cessation, counseling, and vaccination for influenza and pneumococcus.
All patients treated at home should be scheduled for a follow-up visit within 2 days to assess any complication of pneumonia.
The role of corticosteroids remains controversial and may be used in patients who remain hypotensive with presumed adrenal insufficiency.
Distinguishing pneumonia from other pulmonary diseases can be a daunting task, particularly in patients with co-existing pulmonary pathology. The differential diagnoses are different for children and adults, as mentioned below:
Differential Diagnosis in Children
Differential Diagnosis in Adults
Prognosis of pneumonia depends on many factors including age, comorbidities, and hospital setting (inpatient or outpatient). Generally, the prognosis is promising in otherwise healthy patients. Patients older than 60 years or younger than 4 years of age have relatively poorer prognosis than young adults. If pneumonia is left untreated, the overall mortality may become 30%. Antibiotic resistance is very concerning due to the excessive and unjustified use of antibiotics. The Pneumonia Severity Index (PSI) may be utilized as a tool to establish a patient's risk of mortality.
A study conducted on etiologies of CAP, S pneumoniae was found to be the cause of mortality in most patients; however, Pseudomonas, Staphylococcus aureus, and mixed etiologies had the highest mortality rates in those affected. 
The most common complications of bacterial pneumonia are respiratory failure, sepsis, multiorgan failure, coagulopathy, and exacerbation of preexisting comorbidities. Other potential complications of bacterial pneumonia include:
Patients should be counseled to quit smoking, to abstain from alcohol intoxication, and to keep dental hygiene. Furthermore, to prevent bacterial pneumonia, recommendations include:
Elderly and immunocompromised patients should be instructed to seek medical assistance as soon as they develop symptoms such as dyspnea, rigors, or fever.
The management of pneumonia requires an interprofessional team. The reason is that most patients are managed as outpatients, but if not properly treated, the morbidity and mortality are high.
Besides the administration of antibiotics, these patients often require chest physical therapy, a dietary consult, physical therapy to help regain muscle mass, and a dental consult. The key is to educate the patient on the discontinuation of smoking and abstaining from alcohol.
Patients need to be referred to a dietitian to ensure that they are eating healthy.
Further, the clinicians should encourage patients to get appropriate influenza and pneumococcal vaccines. The pharmacist should not only teach about antibiotic compliance but ensure that the patient is prescribed the right antibiotics aimed at the target organism. An infectious disease specialty-trained pharmacist is particularly helpful in assisting the team with difficult antibiotic treatment choices. Nursing can counsel on the appropriate dosing and administration of medications and answer patient questions, as well as charting treatment progress, and reporting any issues to the clinician managing the case.
Finally, it is important to educate the patient to follow up with clinicians if they want a complete resolution of the infectious process. (Level V) Only with open communication between the interprofessional team can the morbidity of pneumonia be lowered.
In healthy people, the outcome after bacterial pneumonia is excellent. However, in people with advanced age, lung disease, immunosuppression, infection with aggressive gram-negative organisms (Klebsiella), and other comorbidities, the outcomes are usually poor. When pneumonia is left untreated, it carries mortality in excess of 25%. Pneumonia can also lead to extensive lung damage and lead to residual impairment in lung function. Other reported complications of pneumonia that occur in 1-5% of patients include lung abscess, empyema, and bronchiectasis. (Level V)
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