Clinical Signs and Symptoms of Influenza

Signs and Symptoms

Uncomplicated influenza

Uncomplicated influenza illness is typically characterized by the abrupt onset of constitutional and upper respiratory tract signs and symptoms (e.g., fever, chills, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis). However, many people who are sick with influenza virus infection do not manifest fever, especially those who are elderly or immunosuppressed. Atypical signs and symptoms of influenza virus infection can occur, including in frail, institutionalized elderly long-term care facility residents. Among young children with influenza, nausea, vomiting or diarrhea may also occur with respiratory symptoms. Uncomplicated influenza signs and symptoms typically resolve after 3-7 days for the majority of people, although cough and malaise can persist for >2 weeks, especially in elderly people and those with chronic lung disease.


Influenza virus infection of the respiratory tract can cause a wide range of complications that can result in severe disease. Certain people are at increased risk for complications from influenza. In young children, otitis media and respiratory complications such as croup, bronchiolitis, and tracheitis can occur. Other complications in children include cardiac (myocarditis and pericarditis), musculoskeletal (severe myositis), and neurologic (encephalopathy, encephalitis, transverse myelitis, and acute disseminated encephalomyelitis). Reye syndrome is associated with influenza (more common with influenza B than influenza A virus infections) and salicylate exposure; however Reye syndrome with influenza is very rare since aspirin use in children with influenza or varicella was recommended against in 1982. In peopleof all ages, influenza can result in dehydration, and exacerbation of underlying chronic medical conditions (e.g. heart failure, myocardial infarction, cerebrovascular accident, diabetes, chronic obstructive pulmonary disease, asthma), Both primary influenza viral pneumonia and secondary invasive bacterial pneumonia (most commonly with Streptococcus pneumoniae, Staphylococcus aureus, methicillin-sensitive or  methicillin-resistant, and group A Streptococcus) can lead to acute lung injury, respiratory failure, acute respiratory distress syndrome, septic shock, and multi-organ failure. Invasive infection with Neisseria meningitidis resulting in meningococcemia and meningitis can follow influenza.

Information about influenza disease burden is available.

Information on currently approved influenza tests, and guidance on interpretation of testing results is available, as is guidance on antiviral treatment. The Infectious Diseases Society of America Influenza Clinical Practice Guidelines provide additional information about complications associated with influenza, influenza testing, interpretation of testing results, and treatment recommendations.

Difference Between Flu and COVID-19

Influenza (flu) and COVID-19 are both contagious respiratory illnesses, but they are caused by different viruses. COVID-19 is caused by infection with a coronavirus named SARS-CoV-2, and flu is caused by infection with influenza viruses. You cannot tell the difference between flu and COVID-19 by symptoms alone because some of the symptoms are the same. Some PCR tests can differentiate between flu and COVID-19 at the same time. If one of these tests is not available, many testing locations provide flu and COVID-19 tests separately.

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