ILI stands for influenza-like illness, a term used by public health agencies to describe a specific combination of symptoms: fever of 100°F (38°C) or higher, plus a cough and/or sore throat. It’s not a diagnosis of the flu itself. Instead, it’s a standardized category that helps track respiratory illness across populations, even before anyone gets a lab test.
Why ILI Exists as a Category
Influenza causes symptoms that look nearly identical to those caused by many other respiratory viruses. RSV, rhinovirus, seasonal coronaviruses, adenovirus, and SARS-CoV-2 can all produce the same fever-plus-cough picture. Because doctors can’t reliably tell these infections apart just by examining you, public health systems needed a consistent way to monitor respiratory illness trends without requiring a lab test for every patient who walks in with a fever and cough.
That’s what ILI does. It groups together all outpatient visits where someone shows up with that specific symptom combination, regardless of which virus is actually responsible. This makes it possible to spot surges in respiratory illness weeks before lab data catches up.
The Exact Definition
The CDC defines ILI as a fever of 100°F or greater along with a cough and/or sore throat. The World Health Organization uses a slightly different version for international surveillance: a measured fever of 38°C (100.4°F) or higher, plus a cough, with symptom onset within the last 10 days. Pediatric criteria at major children’s hospitals typically follow a similar pattern, using a 100.4°F fever threshold plus cough and/or sore throat during flu season.
These definitions aren’t meant to capture every single case of respiratory illness. They’re designed to be consistent enough that data collected in one city, state, or country can be compared meaningfully to data from another.
Most ILI Cases Aren’t the Flu
This is the part that surprises most people. During a typical flu season, only a fraction of ILI cases turn out to be actual influenza when tested. CDC clinical laboratory data from a recent surveillance week showed that just 15.3% of specimens tested were positive for influenza. That means roughly 85% of people who showed up with ILI symptoms had something else entirely.
The U.S. Outpatient Influenza-like Illness Surveillance Network, known as ILINet, explicitly monitors visits for respiratory illness that matches the ILI pattern, not laboratory-confirmed influenza. The system captures illness caused by any pathogen that produces similar symptoms, including influenza, SARS-CoV-2, and RSV.
How ILI Surveillance Works in the U.S.
ILINet is the primary system the CDC uses to track ILI across the country. Healthcare providers report two numbers each week: how many patients came in with ILI symptoms, and how many total patient visits they had. About 70% of participating providers also break their data down by age group. The CDC then calculates the percentage of all outpatient visits that were for ILI.
This percentage gets compared against a national baseline. For reference, the national baseline sits at 3.1%. When the ILI percentage climbs above that number, it signals that respiratory illness activity is elevated. During one recent December surveillance week, the national ILI percentage hit 4.1%, above baseline for the second consecutive week, a signal that respiratory virus season was ramping up. These weekly reports, published as part of the CDC’s FluView system, are how officials decide when to issue public health advisories or recommend increased vaccination efforts.
ILI vs. SARI
The WHO also tracks a more severe category called SARI, or severe acute respiratory infection. SARI uses the same basic criteria as ILI (fever of 38°C or higher plus cough, onset within 10 days) but adds one critical requirement: the patient needs hospitalization. Together, ILI and SARI give health systems a two-tier view of respiratory illness. ILI captures the outpatient, milder end. SARI captures the cases serious enough to land someone in the hospital.
What Happens When You Have ILI Symptoms
If you visit a doctor with ILI symptoms during flu season, the approach depends on how sick you are and whether you’re at higher risk for complications. For people who are hospitalized, have severe or worsening symptoms, or fall into a high-risk group, antiviral treatment is typically started right away, without waiting for lab results. This is because antivirals work best when started within 48 hours of symptom onset, and waiting for a test result can waste that window.
High-risk groups include infants under 12 months, people with chronic lung, heart, kidney, liver, or neurological conditions, those with weakened immune systems, and people with severe obesity. For otherwise healthy outpatients with mild symptoms, the decision to treat often comes down to clinical judgment: how severe the illness looks, how long symptoms have been present, and how likely influenza is given what’s circulating in the community at that time.
Even when antivirals are started after the 48-hour window, they can still provide benefit for patients with severe or progressive illness, so a late start doesn’t necessarily mean treatment is pointless.
Why the Term Matters to You
You’ll most likely encounter “ILI” in news reports during flu season, on public health dashboards, or in your doctor’s notes. Understanding that it’s a surveillance category, not a diagnosis, helps you interpret what those reports actually mean. A headline saying “ILI activity is high” doesn’t mean the flu is everywhere. It means lots of people are showing up to clinics with fever and cough, and some combination of flu, COVID, RSV, and other viruses is responsible. The specific mix varies week to week and region to region, which is why public health agencies track ILI alongside laboratory testing to figure out which viruses are actually driving the numbers.