Influenza Clinical Presentation

Updated: Oct 31, 2022
  • Author: Hien H Nguyen, MD, MS; Chief Editor: Michael Stuart Bronze, MD  more...
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The presentation of influenza virus infection varies; however, it usually includes many of the symptoms described below. Patients with influenza who have preexisting immunity or who have received vaccine may have milder symptoms.

Onset of illness can occur suddenly over the course of a day, or it can progress more slowly over the course of several days. Typical signs and symptoms include the following (not necessarily in order of prevalence):

  • Cough and other respiratory symptoms
  • Fever
  • Sore throat
  • Myalgias
  • Headache
  • Nasal discharge
  • Weakness and severe fatigue
  • Tachycardia
  • Red, watery eyes

Cough and other respiratory symptoms initially may be minimal but frequently progress as the infection evolves. Patients may report nonproductive cough, cough-related pleuritic chest pain, and dyspnea. In children, diarrhea may be a feature.

Fever may vary widely among patients, with some having low fevers (in the 100°F range) and others developing fevers as high as 104°F. Some patients report feeling feverish and feeling chills.

Sore throat may be severe and may last 3 to 5 days. The sore throat may be a significant reason why patients seek medical attention.

Myalgias are common and range from mild to severe. Frontal or retro-orbital headache is common and is usually severe. Ocular symptoms develop in some patients with influenza and include photophobia, burning sensations, or pain upon motion. Some patients with influenza develop rhinitis of varying severity, but it generally is not the chief symptom.

Weakness and severe fatigue may prevent patients from performing their normal activities or work. Patients report needing additional sleep. In some cases, patients with influenza may be bedridden.

The incubation period of influenza averages 2 days but may range from 1 to 4 days in length. Because of aerosol transmission, and the possibility (albeit less likely) of transmission by asymptomatic persons and contaminated surfaces, the patient may be unaware of exposure to the disease. [14]

2009-2010 H1N1 influenza pandemic

In the 2009-2010 H1N1 influenza pandemic, initial symptoms included the following:

  • High fever
  • Myalgias
  • Rhinorrhea
  • Sore throat
  • Nausea and vomiting
  • Diarrhea

For more information, see the article H1N1 Influenza (Swine Flu).




Physical Examination

General appearance varies among patients who present with influenza. Some patients appear acutely ill, with some weakness and respiratory findings, whereas others appear only mildly ill. Upon examination, patients may have some or all the following findings:

  • Fever of 100-104°F; fever is generally lower in elderly patients than in young adults
  • Tachycardia, which most likely results from hypoxia, fever, or both
  • Pharyngitis - Even in patients who report a severely sore throat, findings may range from minimal infection to more severe inflammation
  • Eyes may be red and watery
  • Skin may be warm to hot, depending on core temperature status; patients who have been febrile with poor fluid intake may show signs of mild volume depletion with dry skin
  • Pulmonary findings may include dry cough with clear lungs or rhonchi, as well as focal wheezing
  • Nasal discharge is absent in most patients
  • Fatigued appearance

Acute encephalopathy has been associated with influenza A virus infection. In a case series of 21 patients, Steininger et al described clinical, cerebrospinal fluid (CSF), magnetic resonance imaging (MRI), and electroencephalographic (EEG) findings. [25] Clinical features included altered mental status, coma, seizures, and ataxia. Of patients who underwent further testing, most had abnormal CSF, MRI, and EEG findings.



Primary influenza pneumonia is characterized by progressive cough, dyspnea, and cyanosis after the initial presentation. Chest radiographs show bilateral diffuse infiltrative patterns, without consolidation, which can progress to a presentation similar to acute respiratory distress syndrome (ARDS).

Risks for viral pneumonia involve complex host immune responses and viral virulence. Persons in the third trimester of pregnancy are at higher risk, as they are for other complications of influenza A and B. The elderly, especially nursing home patients, and those with cardiovascular disease, usually are the groups at highest risk; however, particular influenza strains may target younger persons. For example, in the 1918-1919 epidemic, many young adults died of a pneumonia that some experts believe was caused directly by the virus. [9]

Secondary bacterial pneumonia can occur from numerous pathogens (eg, Staphylococcus aureusStreptococcus pneumoniae, and Haemophilus influenzae). The most dreaded complication is staphylococcal pneumonia, which develops 2 to 3 days after the initial presentation of viral pneumonia. Patients appear severely ill, with productive bloody cough, hypoxemia, an elevated white blood cell (WBC) count, and multiple cavitary infiltrates on chest radiography. [26]

A study from Israel found an increase in S pneumoniae bacteremia during regular influenza seasons; in addition, during the 2009-2010 H1N1 influenza pandemic, there were higher rates of S pneumoniae bacteremia among children (but not among adults) and higher rates of S aureus and Streptococcus pyogenes infections in all age groups. [27]

Methicillin-susceptible S aureus (MSSA) and methicillin-resistant S aureus (MRSA) pneumonias have occurred after influenza pneumonia. MRSA pneumonia may be severe and difficult to treat, and deaths have occurred within 24 hours of presentation of pneumonia symptoms.

S pneumoniae or H influenzae pneumonia, if occurring as a complication, usually develops 2 to 3 weeks after the initial symptoms of influenza. These cases can be managed as a community-acquired pneumonia, in accordance with standard antibiotic and admission-discharge guidelines.

Pulmonary aspergillosis is increasingly recognized as a complication of influenza in critically ill adults, occurring relatively early during severe influenza infection. Risk factors leading to disease include damage to the respiratory epithelium, immunomodulatory agents, and use of neuraminidase inhibitors. [28]  

Myositis is a rare complication. This group of patients may develop frank rhabdomyolysis, with elevated creatine kinase levels and myoglobinuria. Myocarditis and pericarditis have been associated with influenza infection. [29]

A review of avian influenza cases in four countries found that the clinical course progressed to ARDS and respiratory failure in 70-100% of patients. [30] The mean time to the development of ARDS was 6 days. Lymphopenia at presentation is a significant predictor of the progression to ARDS and death. [31]

Severe cases of avian influenza may progress to multiorgan failure. In a study of 12 hospitalized patients with confirmed H5N1 influenza, 75% had respiratory failure, 42% had cardiac failure, and 33% had renal failure. [30]