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]

  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]

  […] 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 that may become [weather.com]

  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]

• 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]

  Symptoms include abrupt onset of fever, cough, malaise, myalgia, sore throat, and headache. [health.gov.au]

  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]

• 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]

• Constitutional Symptom

  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]

• Eosinophilic Asthma

  asthma Eosinophilic pneumonia Löffler's pneumonia Tropical (pulmonary) eosinophilia NOS Type 1 Excludes pulmonary eosinophilia due to aspergillosis (B44.-) pulmonary eosinophilia due to drugs (J70.2-J70.4) pulmonary eosinophilia due to specified parasitic [icd10data.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]

  […] 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 usually appears on [weather.com]

  Pneumonia symptoms typically involve coughing with phlegm, fever with chills, absent appetite, confusion, chest pains worsening with breathing or coughing, headaches, shortness of breath, and fatigue. [belmarrahealth.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]

• Dyspnea

  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]

  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]

  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]

• Dry Cough

  fever, sore throat, dry cough, arthralgias, myalgias, thrombocytopenia, relative lymphopenia, and elevated serum transaminases, some findings suggested an alternate diagnosis, e.g., leukopenia, a highly elevated erythrocyte sedimentation rate, highly [ncbi.nlm.nih.gov]

  cough Worsening of symptoms after 12 to 36 hours: fever, breathlessness that may become extreme and 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 [weather.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]

  Infection with influenza most typically results in lung manifestations limited to dry cough and fever, and understanding how the transition to pneumonia occurs could shed light on interventions that reduce mortality. [medicalxpress.com]

  Distinguishing between influenza and pneumonia signs and symptoms Influenza, the flu, generally comes on suddenly and it arrives with a fever, dry cough, chills and sweats, absent appetite, stuffy nose, and head-to-toe body aches. [belmarrahealth.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]

  […] defined in patients with evidence of both acute infection (either reported fever, reported chills, measured temperature 38.2 or 35 C, or an abnormal white blood cell count or differential) and lower respiratory tract symptoms (abnormal breath sounds, tachypnea [journals.plos.org]

• 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]

• 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]

  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]

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

  Pneumonia symptoms typically involve coughing with phlegm, fever with chills, absent appetite, confusion, chest pains worsening with breathing or coughing, headaches, shortness of breath, and fatigue. [belmarrahealth.com]

• 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]

• Cardiomegaly

  At autopsy there was cardiomegaly present but there were no signs of acute myocardial infarction or myocarditis. Pathologically, he died of severe H1N1 pneumonia and not bacterial pneumonia. [ncbi.nlm.nih.gov]

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]

  pulmonary 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 [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 [journalpulmonology.org]

• Bilateral Pulmonary Infiltrate

  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]

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.