Influenza is a rather common viral upper respiratory tract infection among humans. It is caused by influenza A (and less commonly B) viruses, and the pathogenesis, as well as transmission, involves several animal hosts and human-to-human spread through air droplets via inhalation, coughing, or sneezing [1] [2]. After an incubation period of a few days, constitutional symptoms of myalgia, fatigue, malaise, anorexia, fever, and a dry cough are main manifestations of influenza infection and usually last for 7-10 days [1] [3] [4]. It must be noted that the majority of infections are seen during the winter, and rarely in other parts of the year when pandemic or epidemic outbreaks occur [1] [2] [4]. In some patients, the infection is self-limiting, but numerous complications may arise, one of them being influenza pneumonia [1] [2] [3] [4] [5] [6]. It is regarded as the most severe complication and appears more frequently in the presence of various risk factors [5]. In children, concomitant cardiorespiratory illnesses (eg. asthma), diabetes mellitus, immunosuppression, long-term use of aspirin, and other diseases that increase the risk of hypertension are well-established risk factors, whereas pregnancy, chronic diseases, obesity, and residency in chronic care facilities are strongly associated with the development of influenza complications in adults and the elderly [2] [4] [5]. Influenza pneumonia represents the dissemination of the virus into the lower respiratory tract and the lung parenchyma, but secondary bacterial infections of the lungs might also be the culprit in influenza-infected patients, as the virus indirectly promotes bacterial adhesion to the respiratory epithelium [1] [2] [4] [5] [7]. For this reason, a clear distinction between primary influenza pneumonia and secondary bacterial pneumonia may be difficult to make. Nevertheless, approximately 2-5 days after the appearance of signs and symptoms typical for influenza, dyspnea, hypoxia, hemoptysis, cyanosis, and rapidly ensuing acute respiratory distress are hallmarks of influenza pneumonia [2] [3] [4] [8]. In addition, cognitive impairment, gastrointestinal complaints (diarrhea, abdominal pain) and myalgia are reported, and are sometimes the only symptoms, particularly in the elderly population [6]. In fact, pneumonia is one of the most important causes of death from this viral infection, and fatal outcomes might be seen within 24 hours [3] [5] [8] [9].

• Fever

  Getty Images 6/7 Fever Due To Vector Borne Diseases These diseases include malaria, dengue, yellow fever, and chikungunya - all of which have high fever as one of their first symptoms and many others are similar to flu or viral fever. [economictimes.indiatimes.com]

  […] and severe fatigue > A cough that produces greenish or rust-colored mucus These are symptoms of viral pneumonia: > Initial flulike symptoms: fever, headache, muscle aches and dry cough > Worsening of symptoms after 12 to 36 hours: fever, breathlessness [weather.com]

  A 47-year-old man with a fever was highly suspected of having influenza A infection since his wife and son who lived with him had been diagnosed with influenza A. [ncbi.nlm.nih.gov]

  Symptoms include: chills fever hearing loss ear drainage vomiting mood changes An adult with ear pain or discharge should see their doctor as soon as possible. [healthline.com]

• Malaise

  After an incubation period of a few days, constitutional symptoms of myalgia, fatigue, malaise, anorexia, fever, and a dry cough are main manifestations of influenza infection and usually last for 7-10 days. [symptoma.com]

  […] might notice a persistent, productive cough (mucus or phlegm comes out), sometimes with yellow-green phlegm; chest pain, particularly during deep breathing; shortness of breath doing things that wouldn't otherwise leave you winded; overall fatigue and malaise [shape.com]

  However, if they are symptomatic, they typically present with sudden onset of high fever, headache, muscle/joint aches, nonproductive cough, and severe malaise. Inflammatory markers are usually normal or slightly elevated. [amboss.com]

  The clinical features of Mycoplasma pneumonia resemble influenza: The patient has gradual onset of headache, malaise, fever, sore throat, and cough. [mdedge.com]

  The other symptoms, especially cough and malaise, may persist for up to 2 weeks. Unlike adults, children may present with vomiting and diarrhea. [thermofisher.com]

• Acutely Ill Patient

  Two of these variables (C-reactive protein and albumin) were excluded from the derivation of the diagnostic prediction model because they are not routinely performed on admission in all hospitals for acutely ill patients. [thorax.bmj.com]

• Cough

  Conclusions: For outpatient adults with acute cough due to suspected pneumonia, we suggest the following clinical symptoms and signs are suggestive of pneumonia: cough; dyspnea; pleural pain; sweating, fevers, or shivers; aches and pains; temperature [mayoclinic.pure.elsevier.com]

  The Expert Cough Panel agreed on the following clinical symptoms as criteria suggestive of pneumonia in outpatient adults: cough, dyspnea, pleural pain, sweating, shivers, aches, temperature of ≥38°C, tachypnea, and new and localizing chest examination [clinicaladvisor.com]

  But a cough that lasts more than two weeks — or is accompanied by a high fever, shortness of breath, chest pain or coughing up blood — needs prompt attention, particularly during the winter months, when influenza is rampant. [aarp.org]

  You may also experience pain in your ribs from frequent, hard coughing. Coughing – Both the flu and pneumonia cause coughing. A flu cough is likely to be a dry cough. A pneumonia cough is likely to bring up phlegm that has built up in your lungs. [augustahealth.com]

  […] a more severe dry cough that produces small amounts of mucus > A bluish color to the lips; this occurs in some cases These are symptoms of the flu: > Headache > Chills > Dry cough > Muscle aches (myalgia) > Fever > Stuffy nose > Sore throat The fever [weather.com]

• Dry Cough

  […] a more severe dry cough that produces small amounts of mucus > A bluish color to the lips; this occurs in some cases These are symptoms of the flu: > Headache > Chills > Dry cough > Muscle aches (myalgia) > Fever > Stuffy nose > Sore throat The fever [weather.com]

  Although her clinical and laboratory findings were consistent with the diagnosis of H1N1 pneumonia, e.g., fever, sore throat, dry cough, arthralgias, myalgias, thrombocytopenia, relative lymphopenia, and elevated serum transaminases, some findings suggested [ncbi.nlm.nih.gov]

  You may also experience pain in your ribs from frequent, hard coughing. Coughing – Both the flu and pneumonia cause coughing. A flu cough is likely to be a dry cough. A pneumonia cough is likely to bring up phlegm that has built up in your lungs. [augustahealth.com]

  After an incubation period of a few days, constitutional symptoms of myalgia, fatigue, malaise, anorexia, fever, and a dry cough are main manifestations of influenza infection and usually last for 7-10 days. [symptoma.com]

  Learn more Cryptogenic Organizing Pneumonia (COP) Symptoms, Causes and Risk Factors The cause of COP is unknown and in most cases, gradual onset of shortness of breath and dry cough are the most common symptoms. [lung.org]

• Dyspnea

  Case Report: We report a case of a 78-year-old woman admitted to the emergency department in January 2007 for fever and dyspnea. On admission, she had a temperature of 38.8°C and dyspnea. There were wheezing on auscultation. [amjcaserep.com]

  Pure influenza viral pneumonia was observed in 94 cases, which were most frequently young adults with chief complaints of non-productive cough and dyspnea. Radiological imaging showed diffuse ground-glass opacity. [ncbi.nlm.nih.gov]

  Conclusions: For outpatient adults with acute cough due to suspected pneumonia, we suggest the following clinical symptoms and signs are suggestive of pneumonia: cough; dyspnea; pleural pain; sweating, fevers, or shivers; aches and pains; temperature [mayoclinic.pure.elsevier.com]

  The Expert Cough Panel agreed on the following clinical symptoms as criteria suggestive of pneumonia in outpatient adults: cough, dyspnea, pleural pain, sweating, shivers, aches, temperature of ≥38°C, tachypnea, and new and localizing chest examination [clinicaladvisor.com]

  Nevertheless, approximately 2-5 days after the appearance of signs and symptoms typical for influenza, dyspnea, hypoxia, hemoptysis, cyanosis, and rapidly ensuing acute respiratory distress are hallmarks of influenza pneumonia. [symptoma.com]

• Tachypnea

  Conclusions: For outpatient adults with acute cough due to suspected pneumonia, we suggest the following clinical symptoms and signs are suggestive of pneumonia: cough; dyspnea; pleural pain; sweating, fevers, or shivers; aches and pains; temperature ≥ 38°C; tachypnea [mayoclinic.pure.elsevier.com]

  The Expert Cough Panel agreed on the following clinical symptoms as criteria suggestive of pneumonia in outpatient adults: cough, dyspnea, pleural pain, sweating, shivers, aches, temperature of ≥38°C, tachypnea, and new and localizing chest examination [clinicaladvisor.com]

  These factors are being male, thoracic pain, hypothermia, systolic hypotension, tachypnea, diabetes, neurologic disease, cancer, leukopenia, and multilobar infiltration [4, 13, 15, 29]. [hindawi.com]

  Her condition then rapidly deteriorated with development of fever to 38.5°C, hemodynamic instability (sinus tachycardia to 160 beats per minute, blood pressure of 82/53), and respiratory decompensation (tachypnea of 62 breaths per minute, oxygen saturation [frontiersin.org]

  When pneumonia develops, cough, followed by dyspnea, tachypnea, and chest pain, are reported. [medscape.com]

• Rales

  Rales and decreased breath sounds in lung segments are best heard with deep breaths. [mdedge.com]

  Physical examination may demonstrate evidence of conjunctivitis, nasal discharge, hyperemic pharyngeal mucosa without exudate, cervical adenopathy, and, rarely, rales. [thermofisher.com]

  If patients have lower respiratory tract symptoms and signs (eg, dyspnea, rales noted during lung examination), pulse oximetry to detect hypoxemia and a chest x-ray to detect pneumonia should be done. [merckmanuals.com]

  […] alcoholism/alcoholic withdrawal, hyponatremia, unexplained metabolic acidosis, elevated lactate, cirrhosis, asplenia Major Criteria • Invasive mechanical ventilation • Septic shock requiring vasopressors Diagnostic Tests • Physical exam: º Crackles or rales [empr.com]

  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.[20] Crackles (rales) may be heard over the affected area during inspiration.[20 [en.wikipedia.org]

• Chest Pain

  Young healthy adults without a cardiac history who have H1N1 and chest pain usually have either acute myocardial infarction or acute myocarditis. [ncbi.nlm.nih.gov]

  “Both can have the symptoms of coughing, fever and chest pain.” [health.clevelandclinic.org]

  If the doctor diagnoses bacterial pneumonia, treatment typically includes antibiotics and possibly other medications to help relieve coughing and chest pain. [weather.com]

  One person died at home suddenly; the other patients' presenting symptoms were cough with bloody sputum, fever, and pleuritic chest pain. Chest films showed extensive bilateral opacities. [pulmccm.org]

  With pneumonia, there may also be chest pain, sputum production (which may be green, yellow or blood-tinged), difficulty breathing and rapid breathing and pulse rate. [bodyandsoul.com.au]

• Tachycardia

  With H1N1 pneumonia, clinicians should be alert for otherwise unexplained tachycardia or chest pain that may represent acute myocardial infarction or myocarditis. [ncbi.nlm.nih.gov]

  She was ill-appearing in mild respiratory distress with left lower lung field crackles, tachycardia without murmurs, and bilateral lower extremity pitting edema. [frontiersin.org]

  The latter presented more frequently with fever and tachycardia. [erj.ersjournals.com]

• Cyanosis

  After typical symptoms of upper respiratory tract infection and fever appear, severe respiratory decline with hypoxia and cyanosis can ensue without early recognition and appropriate therapy. [symptoma.com]

  […] that usually worsens when taking a deep breath, known as pleuritic pain fast heartbeat fatigue and weakness nausea and vomiting diarrhea sweating headache muscle pain confusion or delirium, especially in older adults dusky or purplish skin color, or cyanosis [medicalnewstoday.com]

  The symptoms may worsen with time, and new respiratory symptoms, such as dyspnea and cyanosis, may appear. This form is the least common but the most severe in terms of pulmonary complications. [medscape.com]

Mortality rates of influenza pneumonia are very high without early therapy, thus a detailed and comprehensive clinical, radiological, and microbiological assessment is vital in preventing the onset of respiratory decline. Firstly, physicians must obtain a complete patient history that will note the course and progression of symptoms and reveal any preexisting comorbidities that could predispose patients to a more severe form of infection. The physical examination is important in guiding the physician toward the lungs as the primary source of infection, after which both imaging studies and microbiological evaluation are necessary. Plain radiography is a valuable tool for visualizing the changes in the lung parenchyma for many infections, including influenza, and bilateral reticular or reticulonodular opacities, as well as patchy areas of consolidation (although other authors suggest that consolidation is absent) are principal features of influenza pneumonia [2] [3] [5] [9]. Unfortunately, not all patients exhibit typical X-ray findings [6]. Thus, confirmation of influenza virus as the underlying cause through microbiological testing is crucial. Several tests are available [1] [2] [5] [6]:

• Rapid influenza diagnostic test (RIDT) - Very fast results (within 15-30 minutes), the ability to discern between influenza A and B, and very high specificity are the reasons why RIDT is quite often performed in standard practice [1] [3] [4] [5].
• Reverse transcriptase-polymerase chain reaction (RT-PCR) - Although it is more expensive and requires advanced technical equipment, the RT-PCR test detects viral genetic material and possesses very high sensitivity and specificity rates [2] [5]. Because of its fast turnaround time (1-4 hours), it is considered to be the optimal diagnostic procedure [3] [5].
• Viral culture - Long waiting times (several days) are the main limitation of viral cultivation in the setting of influenza pneumonia, despite the fact that the specificity rate is virtually 100%. However, newly designed tests can yield conclusive results within 1-3 days [5].
• Serology - Detection of specific influenza antibodies is now being replaced by faster and more reliable methods for the diagnosis of influenza, and serology is often used as a retrospective analysis [5].

• Pulmonary Infiltrate

  Early diagnosis of the pulmonary infiltrates was accomplished by obtaining bronchial secretions with fiberoptic bronchoscopy and staining them with influenza-A fluorescent conjugate. [ncbi.nlm.nih.gov]

  […] eosinophilia due to drugs (J70.2-J70.4) pulmonary eosinophilia due to specified parasitic infection (B50-B83) pulmonary eosinophilia due to systemic connective tissue disorders (M30-M36) pulmonary infiltrate NOS (R91.8)J82) aspiration pneumonia NOS [icd10data.com]

  We have adapted a scoring system described by Opravil to grade the severity of pulmonary infiltrates in CXR: each lung is divided into four equal quadrants and each quadrant is scored on a scale of 0–3 (0: normal, 1: subtle increase interstitial markings [elsevier.pt]

• Bilateral Pulmonary Infiltrates

  The initial chest radiograph may be normal, but several days later, bilateral pulmonary infiltration indicating pulmonary edema develops (55). Pleural effusion is common. [pubs.rsna.org]

The range of absolute risk differences in mortality between the early treatment group and the late treatment group was 26% to 50% (pooled risk difference, 41% [CI, 29% to 54%]). [ncbi.nlm.nih.gov]

Get treatment quickly Early treatment of bacterial pneumonia aids a successful recovery, so see your health care provider as soon as symptoms appear. [weather.com]

The recommended duration for antiviral treatment is 5 days, although longer treatment courses for patients who fail to improve after 5 days of treatment can be considered [ 70 ••]. [link.springer.com]

In outpatient adults with acute cough and suspected influenza, the panel suggests antiviral treatments within 48 hours; antiviral treatments may be linked to lower use of antibiotics, fewer hospitalizations, and optimal outcomes. [clinicaladvisor.com]

In patients with seasonal influenza, treatment was as follows. [erj.ersjournals.com]

Moreover, metabolomics is a highly sensitive and specific tool for the 90-day prognosis of mortality in H1N1 pneumonia. [ncbi.nlm.nih.gov]

Plasma metabolomics for the diagnosis and prognosis of H1N1 influenza pneumonia. Crit. Care. 21:97 (2017) [bioanalysis-zone.com]

Prognosis Prevention Influenza vaccine Recommendation Annual flu shot for all persons aged 6 months and older every flu season as soon as the vaccine becomes available Egg allergy is no longer considered a contraindication Vaccines Live attenuated or [amboss.com]

[…] been described 2 : initial chest radiographs usually show central or peripheral pulmonary ground-glass opacities (GGO) and consolidations that have a patchy or nodular appearance multizonal and bilateral peripheral opacities are associated with adverse prognosis [radiopaedia.org]

Differentiation between a bacterial and viral etiology for the pulmonary infiltrates frequently presents a diagnostic dilemma. Rapid diagnosis is essential; and once established, the patient requires careful supportive therapy. [ncbi.nlm.nih.gov]

When pneumonia is documented, another reason for a query would be to identify the etiology of the pneumonia. Documented etiology could change the MS-DRG assignment. [hcpro.com]

Indications of the Etiology or Etiologies of Pneumonia in Patients with Influenza-Related Lower Respiratory Tract Disease. [nejm.org]

All have agreed as to the uniform character of the disease influenza wherever it has appeared in its swing across the continent, but conflicting views have been presented as to its etiology and as to the relation of various bacteria discovered in the [jamanetwork.com]

Rohani is a professor of ecology and evolutionary biology, a professor of complex systems and a professor of epidemiology at the School of Public Health. [ns.umich.edu]

Research Article Epidemiology Sourya Shrestha 1, 2, *, Betsy Foxman 3, Daniel M. [stm.sciencemag.org]

The recent epidemiology findings have helped to partially resolve the contribution of different pathogens. [journals.lww.com]

Article Figures/Media 22 Citing Articles Article We are now facing a pandemic caused by an epidemiologically distinct, novel virus, the 2009 pandemic influenza A (H1N1) virus (swine flu), against which few persons born since 1970 have antibodies. [nejm.org]

INTRODUCTION: Comparisons of the characteristics between the influenza A (H1N1) pdm09 and common seasonal influenza are important for both clinical management and epidemiological studies. [ncbi.nlm.nih.gov]

Etiology Virus: Influenza virus A and B (and rarely influenza C) Person-to-person transmission: directly via respiratory droplets (sneezing or coughing) or indirectly through contact with contaminated surfaces References:[2][3] Classification Pathophysiology [amboss.com]

Out of this work, the distinction between viral and bacterial strains was noticed.[8] Pathophysiology[edit] Viruses must invade cells in order to reproduce. [en.wikipedia.org]

《Pathophysiology: The Biologic Basis for Disease in Adults and Children》 7판. Elsevier Health Sciences. 498쪽. [ko.wikipedia.org]

The pathophysiology of such conditions is most likely attributable to her overall degree of critical illness as well as bacterial coinfection with MRSA, but is also within the realm of influenza-associated morbidity (6, 15–19), especially in non-vaccinated [frontiersin.org]

The Centers for Disease Control and Prevention (CDC) recommends annual influenza vaccinations for everyone age 6 months or older. [news.vanderbilt.edu]

Prevention To prevent contracting the flu people can take everyday precautions such as regularly washing their hands and avoiding those who are sick, but the best way to prevent the flu is by receiving the flu vaccination every year. [statista.com]

Centers for Disease Control and Prevention (CDC). Available at http://www.bt.cdc.gov/HAN/han00347.asp. Accessed: July 22, 2013. Preventing Seasonal Flu With Vaccination. Centers for Disease Control and Prevention. [medscape.com]

Know the symptoms Prevention is the best medicine, but recognizing pneumonia symptoms and getting professional care right away can help people protect their health. [weather.com]

University of Virginia researchers have discovered two asthma and allergy drugs that may offer a way to prevent a form of pneumonia that is extremely dangerous and sometimes fatal. [ideastations.org]

1. Mandell GL, Bennett JE, Dolin R. Mandel, Douglas and Bennett's Principles and Practice of Infectious Diseases. 8th ed. Philadelphia, Pennsylvania: Churchill Livingstone; 2015.
2. Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. Seventh edition. Philadelphia: Elsevier/Saunders; 2013.
3. Rello J, Pop-Vicas A. Clinical review: Primary influenza viral pneumonia.Crit Care. 2009;13(6):235.
4. Taubenberger JK, Morens DM. The Pathology of Influenza Virus Infections. Annual review of pathology. 2008;3:499-522.
5. Marzoratti L, Iannella HA, Gómez VF, Figueroa SB. Recent Advances in the Diagnosis and Treatment of Influenza Pneumonia. Curr Infect Dis Rep. 2012;14(3):275-283.
6. Falsey AR, Walsh EE. Viral pneumonia in older adults. Clin Infect Dis. 2006;42(4):518-524.
7. Joseph C, Togawa Y, Shindo N. Bacterial and viral infections associated with influenza. Influenza Other Respir Viruses. 2013;7(2):105-113.
8. Rello J, Rodríguez A, Ibañez P, et al. H1N1 SEMICYUC Working Group. Intensive care adult patients with severe respiratory failure caused by Influenza A (H1N1)v in Spain. Crit Care. 2009;13:R148.
9. Kim EA, Lee KS, Primack SL, et al. Viral pneumonias in adults: radiologic and pathologic findings. Radiographics. 2002;22 Spec No:S137-149.