Table 2.

Defining influenza

Influenza symptoms	Illnesses that mimic influenza
• Patient knows exact day that flu hit [25]; symptoms start abruptly, 1–2 days after contagion [4] • Patient has headache, chills, dry cough, myalgia, exhaustion [25]; eye redness [4] • Fever is a frequent early symptom [4] • Illness lasts longer than a cold [25] • Children may experience nausea, vomiting [25]; gastrointestinal symptoms (e.g., diarrhea) are common [4] • Life-threatening complications (e.g., pneumonia) are possible, especially in frail, young, elderly, or chronically sick individuals [4, 15] • Illness is very severe in immunosuppressed individuals [4]	Colds: gradual arrival; runny nose, sneezing; fever absent or mild (fever is a negative predictor of rhinovirus infection in adults) [26]
	Streptococcal pharyngitis: sore throat with nasal symptoms typical of viral pharyngitis, tender unilateral adenopathy and exudate typical of streptococcal pharyngitis (severe sore throat is evidence against influenza) [26]
	Acute mononucleosis: elevated liver function test results, splenomegaly, atypical lymphocytes on peripheral smear, positive monospot test [26]
	Bacterial pneumonia: pleuritic chest pain, productive sputum (illness may be concurrent with viral pneumonia or may occur ≤2 weeks after recovery from influenza) [26]
	Asthma exacerbations, chronic bronchitis, congestive heart failure [27]
	Bacterial meningitis: clouded sensorium, prominent headache (early presentation may be confused with influenza. Influenza should show improvement within 48 h; it is associated with increased risk of invasive meningococcal disease) [26]
	Encephalitis: fever, change in mental status, stiff neck, headache [26]
	Other diseases [many relatively rare conditions can present with influenza-like symptoms (e.g., inhalational anthrax)] [26]