Hey ,
I know a lot of people recommend it – especially in the blogosphere. There are studies that confirm that regular use of saunas can increase white blood cells in healthy adults (according to this study for example)
I believe having saunas can be really beneficial, but you should consider it like the effect of a workout. It is a stressor to which the body adapts positively.
The direct benefit of a sauna is that it can release heat shock proteins. Heat shock proteins can directly stimulate innate immune responses, such as the maturation and activation of dendritic cells and the activation of natural killer cells
But it is also a stressor. We see studies mentioning that a sauna can increase cortisol and catecholamines like adrenaline and noradrenaline (Just like workouts!). And although taking regularly a sauna can increase white blood cells, having a sauna while being sick isn’t being investigated well.
So what does this all mean?
Based on what I have read I would recommend letting you guide by how you feel. If you’re too sick (especially when you have a fever), I would recommend staying in bed. There is a risk of fainting. If you feel like a sauna, I wouldn’t use too high temperatures or stay in too long.
The rule of thumb would be to feel warm, but not so warm that my body needs to fight off the urge to go outside. This is the moment that your body wants to go into a fight-or-flight state. A reason for me to stay out would be if I’m dehydrated because of a fever for example – or having diarrhea.
One last thing: I would also make sure I’m the only one in the sauna since I wouldn’t like to contaminate others.
I hope this makes sense!