Recently I came down with the flu, nothing serious but I always track my core body temperature since it can be: a) necessary if at one point you need to call the doctor and you need to tell about how your fever progressed b) To have a bit of an indication whether you're good to go for sports etc.
This app would be perfect for (manually) tracking core body temperature and could even provide some advice on how long you should recover before attempting sports again.
(20 years ago I had a colleague drop dead in front of me during a teambuilding sports activity, turned out a week earlier he had a heavy case of the flu and decided to go cycling/training as usual. The infection spread to his heart, causing a heart attack. The doctor told me afterwards it is very unwise(dangerous even) to do sports while having a fever caused by an infection. That's why I'm taking this seriously.