Time in the tub may improve your immune system
Hot Tub do not get you all the way there, but warming up in one is a start. You can then put very warm towels around your neck to help with the immune response.
There is also evidence that switching between a warm bath, then a cold shower ramps up immunity.
This is NOT medical advise and you need to consult your physician.
Below is a video from a doctor talking about fevers and its effect on COVID-19.
