I've it's quite common for fitness trackers to predict the onset of sickness from heart rate variability. I've found they don't offer much real-time feedback, and don't have breathing detection though which was what I wanted to explore for this project, to train hrv
I can tell I'm sick by looking at my resting heart rate. If it goes up (generally by 5-10 bpm) and I didn't do a hard workout in the past 12-18 hours, I'm pretty likely to be coming down with something.
Is this a feature that some trackers offer? When I worked at Fitbit I noticed this correlation (along with resting heart rate) and developed a model that could predict onset of illness a few days in advance with fairly good accuracy. But unless the accuracy is perfect it can be difficult to offer it as a product feature without a lot of validation and experiments showing that your health related claim is accurate.
