Influenza Symptoms
Influenza is characterized by the abrupt onset of fever, myalgia, headache, severe malaise, nonproductive cough, sore throat, and rhinitis, with symptoms appearing suddenly after a 1-4 day incubation period (average 2 days). 1, 2
Core Constitutional Symptoms
The hallmark presentation includes:
- Fever (typically high-grade, though defined as >100°F in adults and >97.5°F in elderly) 1, 3
- Myalgia (muscle aches throughout the body) 1, 2
- Headache 1, 2
- Severe malaise and fatigue 1, 2
- Chills or rigors/sweats 2, 3
Respiratory Symptoms
- Nonproductive cough (dry cough without mucus production) 1, 2
- Sore throat 1, 2
- Rhinitis (nasal congestion and discharge) 1, 2
- Nasal stuffiness 2
Age-Specific Presentations
Children
- Otitis media (ear infection occurs in approximately 25% of children under 5) 1, 2
- Nausea and vomiting 1, 2
- High fever with conjunctivitis 2
- Irritability and appearing toxic 2
Neonates and Infants
- Non-specific sepsis-like presentation including pallor, floppiness, lethargy, poor feeding, and apnea episodes 2
Elderly (≥65 years)
Clinical Course and Duration
- Uncomplicated illness typically resolves after 3-7 days 2
- Cough and malaise can persist for >2 weeks even after other symptoms resolve 1, 2
- Adults are contagious from 1 day before symptoms through approximately 5 days after illness onset 1, 2
- Children can be infectious for >10 days and may shed virus for up to 6 days before symptom onset 1, 2
Important Clinical Caveats
Influenza cannot be reliably distinguished from other respiratory infections based on symptoms alone. 1, 5 The positive predictive value of fever and cough for laboratory-confirmed influenza ranges from only 30% in elderly patients to 79-88% in healthy adults. 5 This means that clinical diagnosis without laboratory confirmation will miss many cases or misdiagnose other respiratory infections as influenza.
Warning signs requiring immediate medical evaluation include:
- Fever persisting beyond 4 days (suggests complications) 2
- Respiratory distress, cyanosis, or breathing difficulties 2
- Severe dehydration 2
- Altered consciousness or mental status changes 2
- Initial improvement followed by fever recurrence (hallmark of bacterial superinfection) 2
Complications to Monitor
- Primary influenza viral pneumonia 1, 2
- Secondary bacterial pneumonia (occurs in 20-38% of severe cases, most commonly Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae) 2
- Exacerbation of underlying chronic conditions (cardiac, pulmonary disease) 1, 2
- Encephalopathy or encephalitis (in children) 2
- Acute respiratory distress syndrome 2