Introduction Hippocrates (460–370 BC) was the first to describe pneumonia (1). Pneumonia is a lung illness caused by some microorganisms. Pneumonia is an infection of the pulmonary parenchyma caused by various organisms. It’s a lung infection caused by an acute respiratory tract infection (ARTI). When a person gets pneumonia, the alveoli in their lungs get clogged with pus and fluid, making breathing difficult and restricting oxygen intake (2). Many attempts have been made to define pneumonia based on the etiology, clinical environment in which the patient contracted the infection, and pattern of lung parenchyma involvement, among other factors (2).

Classification of Pneumonia bases on Clinical Environment

1. Community-Acquired Pneumonia (CAP)

Any pneumonia acquired outside of a hospital in a community setting (3).

1. Hospital-Acquired Pneumonia (HAP)

HAP is defined as pneumonia acquired 48 hours after being admitted to an inpatient facility, such as a hospital, and which was not incubating at the time of admission. The distinction between healthcare-associated and hospital-acquired pneumonia is clarified by this classification.  All pneumonia obtained at assisted-living institutions, rehabilitation centers, and other healthcare facilities is now classified as community-acquired pneumonia, and a hospital setting is no longer required for pneumonia to be classified as HAP (4).

Classification of Pneumonia bases on Etiology                                                                                                       These classifications are based on the identification of the common organisms that cause each type of pneumonia. Pneumonia has many possible causes, but the most common are bacteria and viruses (2).

1. Bacterial causes

In terms of ease of culture positivity, they have traditionally been examined under the subheadings “typical” and “atypical” organisms. Pneumococcus, Haemophilus influenzae, Moraxella catarrhalis, Group A Streptococcus, and other aerobic and anaerobic gram-negative organisms are common typical species. Legionella, Mycoplasma, and Chlamydia are among the atypical organisms regularly seen in clinical practice (2). Streptococcus pneumonia                                                                                                                                        The upper respiratory tract is frequently colonized by the pathogen.  The sole natural reservoir for S. pneumoniae is the human nasopharynx, and these bacteria, as well as viruses, are usually discovered in a child’s nose or throat; these infections are subsequently aspirated into the lungs, causing illness. Pneumonia can be transmitted in a variety of ways. The pathogen is spread by direct contact with respiratory secretions, colonizes the nasopharynx, and eventually can cause blood-borne illnesses. S. pneumoniae can cause both non-invasive and invasive diseases in people of all ages, but especially in children under the age of five and adults aged 65 and higher (6). Mycoplasma are the smallest free-living disease pathogens in humans, possessing both bacterial and viral features. “Walking pneumonia”, which occurs predominantly in the summer and fall, is caused by these microorganisms. Patients who are not unwell enough to stay in bed or seek medical attention may be unaware that they have pneumonia. Mycoplasmal pneumonia is widespread among children in daycare centers, as well as school-aged children and young people (7).  Haemophilus influenzae It’s most frequent in the winter and early spring, and it’s usually the result of an upper respiratory illness. Debilitated hosts are also associated with this pneumonia. Risk factors include young age, elderly age, asthma, chronic obstructive pulmonary disease (COPD), smoking, and a weakened immune system (8). Intravenous drug users, those with debilitating conditions (particularly cystic fibrosis), patients with prosthetic devices, hospitalized patients, and residents in chronic care homes are all susceptible to bacterial pneumonia caused by Staphylococcus aureus. In patients who take intravenous medications, the illness is most likely transferred through contaminated injections sites through the bloodstream to the lungs (6).

1. Viral causes

Viruses are frequently seen colonizing the nasopharynx of CAP patients. It’s still being researched whether they’re the major reason or if secondary bacterial factors contribute to the pathogenesis. The influenza virus is the most common viral agent implicated in CAP, followed by respiratory syncytial virus, parainfluenza virus, and adenoviruses (8).

1. Fungal causes

Patients with specific predisposed immunocompromised states, such as HIV and organ transplant recipients, are frequently found to have fungal infections. Some fungal species, on the other hand, can induce pneumonia in immunocompetent people, resulting in a delay in diagnosis and adverse consequences. Histoplasma, Blastomyces, and Coccidioides are the three most common (7). Clinical Diagnosis and clinical manifestation A diagnosis of bacterial pneumonia is based primarily on chest x-ray findings, white blood cell count, and a sputum culture together with fever, recurrent chills, cough, shortness of breath (i.e., dyspnea), and abnormal chest sounds (6).

Sputum examination provides an accurate diagnosis in approximately 50% of patients. A gram stain of expectorated sputum helps to distinguish bacterial from viral pneumonia and gram-negative from gram-positive microbes (6). Sputum samples that result from deep cough provide the best results. A single predominant microbe present with the gram stain is indicative of bacterial pneumonia. Mixed flora may be observed with anaerobic infections. Blood cultures are positive in only approximately 40% of cases, but, when positive, results confirm a causative agent. Cultures may require 24–48 hours incubation before results are available. When there is fluid within the pleural space, transthoracic needle aspiration (i.e., thoracentesis) with examination and culture of the fluid is performed to assist in identification of the causative microorganism (6).  The clinical presentation of bacterial pneumonia varies from a mildly ill, ambulatory patient to a critically ill patient with respiratory failure or septic shock. The sudden onset of symptoms with rapid progression of the illness is typical of bacterial pneumonia. A thorough past medical history and history of potential exposures are usually obtained. If the patient has been healthy previously, the cause of pneumonia is usually Mycoplasma or a gram-positive microbe. However, if the patient has been hospitalized or has a chronic illness, gram-negative microbes are suspected. Although not diagnostic of a particular causative agent, characteristics of the sputum may suggest a particular pathogen. Pneumococci may cause bloody or rust-colored sputum (7). Patients with Pseudomonas, Haemophilus, or pneumococcal infections are known to expectorate green sputum. Anaerobic infections typically produce foul-smelling and bad tasting sputum. Klebsiella and type 3 pneumococci cause the production of thick, dark red sputum (7). Physical examination findings vary depending on the type of microorganism, severity of pneumonia, age of the patient, coexisting host factors, and presence of complications and may include the following: fever or hypothermia, shallow breathing, tachycardia or bradycardia, cyanosis, decreased breath sounds, crackles (rales) with auscultation of the lungs, egophony on auscultation, pleural friction rub, dullness of the chest to percussion, altered mental status (especially confusion) (7). Treatments Around 85% to 90% of antibiotic consumption occurs in the community, with 80% of this consumption going towards treating respiratory tract infections (6). Once a child develops pneumonia, death is avoidable through cost-effective and life-saving treatment from antibiotics for bacterial pneumonia. When children suffering from pneumonia are treated promptly and effectively with antibiotics their chances of survival increases significantly. The most common antibiotics currently recommended for children younger than five years are cotrimoxazole and amoxicillin. Children aged 2-59 months with non-severe pneumonia can be treated with oral amoxicillin (40 mg/kg/dose) for three days and five days for severe pneumonia  For very severe pneumonia, parenteral ampicillin (or penicillin) and gentamicin are recommended as a first line treatment; ceftriaxone should be used as a second line treatment when the first line treatment fails (9).

Vaccination is a safe, effective, and cost-effective tool for preventing pneumonia. There are vaccines against major infectious diseases that can cause pneumonia –the flu (influenza virus), measles, pertussis, Hib, and pneumococcus (9). Immunizations help reduce childhood pneumonia in two ways. First, vaccinations help prevent children from developing infections that directly cause pneumonia, such as Hib and S. pneumoniae . Second, immunizations may prevent infections that can lead to pneumonia as a complication, such as influenza, measles, and pertussis (6). Pneumococcal conjugate vaccines are highly effective in preventing pneumococcal disease. Currently, there are three vaccines on the children’s routine immunization schedule that have the potential to significantly reduce childhood mortality from and related to pneumonia: measles, Hib, and pneumococcal conjugate vaccines (8). Haemophilus influenza type b (Hib) is the second leading cause of bacterial pneumonia in children, but it is preventable with the highly effective Hib vaccine. The Hib vaccine has been shown to have protective efficacy greater than 90% against both laboratory-confirmed invasive meningitis and bactaeremic and non-invasive pneumonia (6). References

1. Hippocrates, (1891). Recent Literature The Genuine Works of Hippocrates . Translated from the Greek, with a Preliminary Discourse and Annotations. By Francis Adams, LL.D., Surgeon. The Boston Medical and Surgical Journal, 125(6), pp.147–147.
2. Mackenzie, G. The definition and classification of pneumonia. Pneumonia (Nathan). 2016;8:14.
3. Bartlett, J.G., Dowell, S.F. and Mandell, L.A. (2000) Practice guidelines for the management of community-acquired pneumonia in adults. Infectious Diseases Society of America. Clinical Infectious Diseases; 31: 347–82.
4. Kalil, A.C., Metersky, M.L., Klompas, M., Muscedere, J., Sweeney, D.A., Palmer, L.B., Napolitano, L.M., O’Grady, N.P., Bartlett, J.G., Carratalà, J., El Solh, A.A., Ewig, S., Fey, P.D., File, T.M., Restrepo, M.I., Roberts, J.A., Waterer, G.W., Cruse, P., Knight, S.L. and Brozek, J.L. (2016). Executive Summary: Management of Adults With Hospital-acquired and Ventilator-associated Pneumonia: Clinical Practice Guidelines by the Infectious Diseases Society of America and the American Thoracic Society. Clinical infectious Diseases; 63(5): 575-82.
5. Gharib, A.M. and Stern, E.J. (2001). Radiology of pneumonia. Medical Clininic North Americal; 85(6): 1461-91
6. Torres, A., Lee, N., Cilloniz, C., Vila, J. and Van der Eerden, M. (2016). Laboratory diagnosis of pneumonia in the molecular age. European Respiratory Journal; 48(6): 1764–1778.
7. Metlay, J.P., Waterer, G.W., Long, A.C., Anzueto, A., Brozek, J., Crothers, K., Cooley, L.A., Dean, N.C., Fine, M.J., Flanders, S.A., Griffin, M.R., Metersky, M.L., Musher, D.M., Restrepo, M.I. and Whitney, C.G. (2019). Diagnosis and Treatment of Adults with Community-acquired Pneumonia. An Official Clinical Practice Guideline of the American Thoracic Society and Infectious Diseases Society of America. American Journal of Respiratory and Critical Care Medicine, 200(7), pp.e45–e67.
8. Hart, C.A. (2004). Microterrors. the complete guide to bacterial, viral, and fungal infections that threaten our health. Buffalo, N.Y.: Firefly Books.
9. Mandell, L.A., Wunderink, R.G., Anzueto, A., Bartlett, J.G., Campbell, G.D., Dean, N.C., Dowell, S.F., File, T.M., Musher, D.M., Niederman, M.S., Torres, A. and Whitney, C.G. (2007). Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults. Clinical Infectious Disease; Suppl 2:S27-72.