What is Influenza-Like Illness?

Influenza-like illness (ILI) is a not a single disease, but is a large group of pathogens grouped together because they have similar symptoms. In the United States, ILI affects an estimated 9-49 million people every year. The CDC defines the presence ILI as a fever of at least 100 degrees Fahrenheit, a cough, and/or a sore throat. ILI is traditionally reported and tracked as a single pathogen; a small percentage of clinical cases are tested to confirm the presence of an underlying virus, bacteria, or fungus. Our question in this study was: how do respiratory viruses with similar symptoms differ in their epidemiological parameters and outbreak properties? We reviewed 104 studies for parameter values of five common seasonal respiratory viruses. We developed a mechanistic model, simulated epidemics for each virus during a hypothetical flu season, and carried out global sensitivity analyses. Of the viruses reviewed, adenovirus has the longest mean incubation period and RSV has the highest R₀. Historic outbreak coronaviruses (SARS and MERS) have almost double the mean incubation period and double the infectious period of the seasonal coronaviruses. Given our model assumptions and initial conditions, our numerical simulations indicate that RSV peaks first, while influenza and SARS/MERS peak approximately five months into the season. In view of these differences, improved ability to distinguish between respiratory pathogens has the potential to increase diagnostic accuracy and pandemic preparedness.