The Finns are really big on saunas, so there could be something to this.
I have personally used Hydrotherapy, and recovery time for any illness is reduced.
It can be used when ill, or for invigoration, on arthritis, against depression and so forth. For instance:
Hot - Cold Shower therapy, invigorates the nervous system, releases white blood cells to fight infection, opens up blood vessels and so on.
3 Minutes Hot Shower as one can take it (do not burn yourself, not trying to make 'you-stew')
30 seconds straight cold (15 sec front, 15 sec back)
3 Minutes Hot again (1 min. 30 front, 1 min. 30 back, roughly)
30 seconds straight cold
3 Minutes Hot again
30 seconds cold
Three alternations, as above, is the minimum. Can do up to 7 times if necessary, or if hot water lasts (mine usually runs out at about 4-5).
Just a slight caution. The change in water temp may cause a slight sensation of dizziness, since the blood vessels are opening and closing depending on the water change, so make sure if you feel this, place your hand against the shower wall to be stable until this feeling is over, or sit in a bath-chair. This may not always happen, but will be more prevalent in head sickness cases. You can run the cold and hot on the head and face for nasal and sinus pressure. However a slight caution for the hot water, it may dry out the face a little if kept too long there, so do not keep the face too long directly in the heat in this instance.
Any showerhead will work, normally, pulsing, straight flow, etc.
End on "cold" straight. You will not feel cold when done, but the air around you will feel nice and even.
Take a dry towel and dry rub with some light friction the body.
Get into body length (evenly clad) clothing (keeps blood circulation even, exposed limbs the heart works harder to get blood to them, and can chill easily).
Drink a full glass of water (as needed)
Take a nap in a quiet place (as needed), you will feel really relaxed.
This works for flu, and other illnesses, or depression, head colds, or if just a little fatigued, or even if just needing a little invigoration.
The first 30 sec cold is the worst (and I usually count out loud, 1001, 1002 ... 1030). Brace for it. After that, it will feel great.
Let me know if any try this, and how they felt after. They used to teach these things in the medical field, but it is rarely taught these days (pity).