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.”^[1]

• 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.

• 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

• Nausea
• Vomiting
• Diarrhea
• Reddened eyes, skin (especially face), mouth, throat and nose
• Shortness of breath (especially in asthmatic patients)^[4]
• Loss of appetite
• Sweating
• Muscle stiffness
• Hemoptysis^[4]

Since anti-viral drugs are effective in treating influenza if given early (see treatment section, below), it can be important to identify cases early. A systematic review by the Rational Clinical Examination concluded that the best findings for excluding the diagnosis of influenza are:^[5]

Using the symptoms listed above, the combinations of findings below can improve diagnostic accuracy.^[8] Unfortunately, even combinations of findings are imperfect. Studies of combining symptoms have included use of recursive partitioning; however, this study combined two cohorts with different prevalences of influenza.^[9] However, Bayes Theorem can combine pretest probability with clinical findings to adequately diagnose or exclude influenza in some patients. The pretest probability has a strong seasonal variation; the current prevalence of influenza among patients in the United States receiving sentinel testing is available at the CDC.^[10] Using the CDC data, the following table shows how the likelihood of influenza varies with prevalence:

Per the bottom row of the table above, a score of 3 or more from^[11]:

• New or increased cough (2 points)
• Headache (1 point)
• Subjective fever (1 point)
• Triage temperature >100.4°C (1 point)