What is Influenza-Like Illness (ILI)?
Influenza-like illness (ILI) is defined as the presence of fever and new (or worsening) cough of acute onset when influenza is circulating in the community. 1, 2
Core Clinical Definition
The essential components of ILI are:
- Fever (typically 38-40°C, though may reach 41°C) 1, 2
- New or worsening cough of acute onset 1, 2
- Context of community influenza circulation 1, 2
This triad of fever, cough, and acute onset represents the most predictive clinical features for influenza infection. 2
Common Associated Symptoms
While not required for the definition, ILI frequently includes:
- Myalgia (present in ~53% of cases), affecting mainly the back and limbs 1, 2
- Headache 1
- Sore throat 1
- Chills or sweats 3
- Malaise and fatigue 1, 3
- Nasal congestion (present in ~91% of influenza cases) 4
Gastrointestinal symptoms like vomiting and diarrhea are uncommon in adults (<10%). 1, 2
Clinical Presentation Timeline
The typical course follows this pattern:
- Incubation period: 2-4 days (range 1-7 days) 1
- Onset: Abrupt, with fever peaking within 24 hours 1
- Fever duration: Typically 3 days (range 1-5 days) 1
- Overall illness: Usually resolves in 7 days, though cough and malaise may persist for weeks 1
Diagnostic Accuracy and Limitations
ILI is not specific for influenza infection—other respiratory pathogens can present identically. 2 The predictive value varies significantly:
- During interpandemic periods: ILI correlates with laboratory-confirmed influenza in only 25-45% of cohort studies 1, 2
- During clinical trials: Correlation reaches 70% 2
- During pandemics: Predictive value increases substantially when a pandemic strain is circulating locally 1
The best clinical predictor is the combination of cough and fever together, which has a positive predictive value of 79% when influenza is circulating in the community. 4 This predictive value increases with higher fever temperatures. 2, 4
Key Differentiating Features
The presence of a rash, lymphadenopathy, or central nervous system symptoms suggests influenza is less likely and alternative diagnoses should be considered. 5 Other respiratory viruses that can mimic ILI include:
- Rhinovirus (accounts for ~20% of ILI cases during influenza season) 6
- Adenovirus 5, 6
- Respiratory syncytial virus 5, 6
- Enterovirus 5, 6
Clinical Pitfalls
A critical caveat: diagnostic accuracy of ILI is higher in persons aged 60 years and above compared to younger patients. 2 Additionally, during influenza season, rhinovirus peak activity often coincides with influenza circulation, creating diagnostic overlap that must be considered in clinical management. 6
The time course of symptoms matters—cough remains the most predictive symptom throughout the illness week, while upper respiratory symptoms (sore throat, productive cough) and general symptoms (body ache, fatigue) are more predictive in the first 2-3 days of illness. 7