Influenza history and symptoms

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For more information about non-human (variant) influenza viruses that may be transmitted to humans, see Zoonotic influenza

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Alejandro Lemor, M.D. [2]

Overview

Influenza illness can include several non-specific symptoms, such as fever, muscle aches, headache, lack of energy, dry cough, sore throat, and runny nose. The fever and body aches can last 3-5 days and the cough and lack of energy may last for 2 or more weeks. The symptoms of the flu are more severe than their common-cold equivalents.

Influenza-like illness (ILI) t is defined as 'a fever (temperature ≥100.0°F [≥37.8°C], oral or equivalent) and cough and/or sore throat, without a known cause other than influenza."

History and Symptoms Adapted from CDC

• The typical incubation period for influenza is 1-4 days.
• Adults shed influenza virus from the day before symptoms manifest through 5-10 days after their onset. However, the amount of virus shed, and presumably infectivity, decreases rapidly by 3-5 days after onset in an experimental human infection model.
• Young children also might shed virus several days before illness onset, and children can be infectious for 10 or more days after onset of symptoms.
• Severely immunocompromised persons can shed virus for weeks or months.
• Uncomplicated influenza illness is characterized by the abrupt onset of constitutional and respiratory signs and symptoms, such as fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis.
• Among children, otitis media, nausea, and vomiting also are commonly reported with influenza illness.
• Uncomplicated influenza illness typically resolves after 3-7 days for the majority of persons, although cough and malaise can persist for >2 weeks.
• However, influenza virus infections can cause primary influenza viral pneumonia; exacerbate underlying medical conditions such as pulmonary or cardiac disease lead to secondary bacterial pneumonia, sinusitis, or otitis media; or contribute to coinfections with other viral or bacterial pathogens.
• Young children with influenza virus infection might have initial symptoms mimicking bacterial sepsis with high fevers, and febrile seizures have been reported in 6%-20% of children hospitalized with influenza virus infection.

Image courtesy of Häggström, Mikael. "Medical gallery of Mikael Häggström 2014". Wikiversity Journal of Medicine 1

Common Symptoms

• Fever (seen in the majority of patients)
• Cough (seen in the majority of patients)
• Chills
• Dizziness
• Flushed face
• Myalgia
• Sneezing
• Rhinorrhea
• Fatigue
• Headache
• Conjunctivitis
• Nasal congestion
• Sore throat

Less Common Symptoms

• Nausea
• Vomiting
• Diarrhea
• Reddened eyes, skin (especially face), mouth, throat and nose
• Shortness of breath (especially in asthmatic patients)
• Loss of appetite
• Sweating
• Muscle stiffness
• Hemoptysis