If you've ever had the flu, you'll know it comes with a host of symptoms that are pretty unpleasant. A pounding heart, high temperature, achy muscles - it can be a nasty experience.
But research out of the US shows that a wearable fitness device could be tracking all that, and using it to predict flu trends across the world.
It might even know you're sick before you do.
In a new study published in The Lancet Digital Health journal, researchers found that heart rate and sleep data from fitness watches can predict and alert public health officials to real-time outbreaks of flu more accurately than current surveillance methods.
Data from more than 200,000 Fitbit users across five US states was released to the US Centre for Disease Control (CDC) during the study period of March, 2016 - March, 2018. By incorporating data from Fitbit trackers, flu predictions at the state level were improved - meaning supplies of vaccines and antivirals were higher, and health professionals could better advise on when to stay home.
Previous studies using Google Trends and Twitter have had varying levels of success, which experts say is down to the simple fact you can't tell the difference between those with the flu googling their symptoms and those who search online about it due to media coverage.
Your resting heart rate tends to spike during infectious episodes, and sleep quality tends to decrease - all things a wearable fitness device can record.
While researchers used Fitbit for the study, the findings could be applied to other popular wearable digital devices like Garmin, or the Apple Watch.
Dr Cécile Viboud from the National Institutes of Health says the study is a "promising first step towards integrating wearable measurements in predictive models of infectious diseases".
"We anticipate that the large amount of real-time data generated by Fitbit and other personal devices will prove highly useful for public health and augment traditional surveillance systems."
WHO estimates that Influenza results in 650,000 deaths worldwide annually. Approximately seven percent of working adults and 20 percent of children aged under five years get the flu each year.
