Flu vs. Influenza-Like Illness (ILI)
The CDC defines the flu as “a contagious respiratory illness caused by influenza viruses that infect the nose, throat and lungs” and says symptoms include some or all of the following:
Fever, Cough and sore throat, Runny or stuffy nose, Muscle or body aches and chills, Headaches, Fatigue
The flu is diagnosed with a test -- a swab of your nose or throat. The CDC says that with ILI, you have a fever (temperature of 100°F or higher) and a cough or sore throat without a known cause.
“To say someone has influenza requires they be tested and have a positive result. In the absence of testing, ILI is what we call a clinical definition. You can make the diagnosis by talking to someone or examining them. It’s easier to do and free because it requires no testing,” explains Ephraim Tsalik MD, PhD, an infectious disease doctor at Duke University Medical Center in Durham, NC./.../
So “flu-like illness” is a wide-ranging category that may involve other viruses with similar symptoms or patients who probably have the flu but didn’t get a flu test.
“You don’t know exactly what the diagnosis is, and a test hasn’t been done that explains (it),” adds Schaffner.