• respiratory infection caused by the influenza virus


  • influenza is a member of the Orthomyxoviridae family

  • influenza A and B are the 2 types of influenza viruses that cause epidemic human disease

  • influenza C is a third subtype that causes mild respiratory illness but is not thought to cause epidemics


  • direct human-to-human transmission occurs largely via large-particle respiratory droplets

    • requires close contact between infected and susceptible individuals

    • large-particle droplets travel ≤6 feet

  • alternative modes of transmission

    • indirect transmission via hand contact with influenza-contaminated surface and subsequent transfer to nose or mouth

    • airborne transmission via small-particle aerosols over short distances

  • duration of viral shedding varies with host factors

    • adults shed virus before symptoms present and continue through 5-10 days following onset of illness

    • children may be infectious for ≥10 days

    • severely immunocompromised persons can shed virus for months

  • temperate countries of northern and southern hemispheres have peak activities in winter months 

Clinical presentation 

  • pattern of illness varies from mild respiratory illness (similar to common cold) to severe prostration without characteristic signs and symptoms

  • common symptoms include

    • fever

    • myalgia

    • arthralgia

    • anorexia

    • headache

    • dry cough

    • malaise

    • fatigue

    • weakness

    • chest discomfort

  • less frequently, symptoms may also include

    • nasal congestion

    • sneezing

    • sore throat

  • signs, symptoms, or complications that may be present in severe or progressive stages of infection

    • central nervous system abnormalities including altered mental status, drowsiness, difficult to awaken, recurring or persistent convulsions, confusion, severe weakness or paralysis

    • cardiopulmonary insufficiency indicated by shortness of breath, dyspnea, tachypnea, cyanosis, bloody or colored sputum, chest pain, low blood pressure, or hypoxia

    • severe dehydration with decreased activity, dizziness, decreased urine output, or lethargy

    • signs and symptoms of secondary complications including bacterial infection, renal or multi-organ failure, septic shock, rhabdomyolysis, myocarditis

    • exacerbation of underlying chronic disease


  • diagnosis made clinically with confirmatory testing in selected cases

    • accuracy of clinical diagnosis on basis of symptoms alone is limited due to overlap of symptoms of other illnesses

    • once influenza activity has been documented in community or geographic area, clinical diagnosis of influenza without testing is acceptable, especially during periods of peak influenza activity

    • diagnostic testing should be considered if
      • results would change management, such as deciding whether to use influenza antiviral agents

      • patients are hospitalized with suspected 

      • institutional outbreak of influenza suspected (to allow prompt implementation of control measures)

  • diagnostic tests available for influenza include rapid antigen detection tests, viral culture, serology, reverse transcriptase-polymerase chain reaction (RT-PCR), and immunofluorescence assays

    • rapid antigen detection tests may provide results early enough to guide individual patient management
      • a negative rapid antigen test does not exclude influenza in symptomatic patients, and antivirals should not be withheld if test is negative and influenza is suspected

      • sensitivity and specificity of tests vary between laboratories, types of tests, specimen source, and timing of specimen collection from illness onset

      • results should be interpreted in the context of other clinical and epidemiologic information

    • consider confirmatory testing by RT-PCR or culture when rapid antigen test is negative and clinical suspicion is high

Differential diagnosis 

  • upper respiratory infection

  • pneumonia

  • acute bronchitis

  • acute exacerbation of chronic obstructive 
pulmonary disease

  • asthma exacerbation

  • Streptococcal pharyngitis

  • acute sinusitis in adults

  • allergic rhinitis

  • infectious mononucleosis

  • acute myeloid leukemia

  • acute HIV infection

  • malaria

  • babesiosis

  • recent vaccination with live, attenuated 
influenza vaccine