CDC FluView / ILINet

What It Measures

ILINet (Influenza-like Illness Surveillance Network) tracks the percentage of outpatient healthcare visits where patients present with influenza-like illness (ILI) — defined as fever (100°F or higher) plus cough or sore throat.

How It Works

Approximately 3,000 healthcare providers across the United States, Puerto Rico, the U.S. Virgin Islands, and the District of Columbia voluntarily report the total number of patient visits and the number of visits for ILI each week. CDC FluView can still have current state-level gaps in the ILINet rows Local Health Signal uses, so some state pages explain the missing signal instead of showing a fake low number.

What It Tells You

The ILI percentage shows how common flu-like symptoms are relative to all doctor visits. A higher percentage means more people are seeking care for flu symptoms. The metric is useful for tracking trends over time and comparing across regions.

Limitations

• ILI is a syndrome, not a diagnosis. Patients counted as ILI visits may have flu, COVID-19, RSV, or other respiratory infections.
• Not all providers report every week, so data can be revised.
• Some states have better provider coverage than others, and some weekly state rows may be missing from the current feed.
• The data reflects who goes to the doctor — people who stay home sick are not captured.

How We Access It

We access ILINet data through the Delphi Epidata API, which provides a programmatic interface to CDC FluView data. This allows us to automate our weekly data pipeline.