Does sauna bathing help against infections?

Regular sauna bathing may reduce the severity of flu and respiratory infections. That signal comes from observational studies and narrative review articles, not from large randomised trials. These are therefore early indications, not an established clinical benefit.

A small study in 20 healthy young men showed that immune system changes only became measurable after a series of ten sauna sessions: there were shifts in white blood cells and certain types of lymphocytes. A single sauna session did not produce that effect. In addition, trained and untrained men responded differently, so it is not a given that this applies equally to everyone.

Sweat naturally contains certain protective proteins that can inhibit bacteria on the skin. Whether sauna bathing raises the concentration of those proteins compared with ordinary sweating cannot be determined from the available studies.

Animal research in mice and hamsters shows that an elevated body temperature can increase resistance to influenza virus and SARS-CoV-2, through a mechanism linked to the gut microbiome. This has so far only been demonstrated in animals; whether sauna bathing produces the same effect in humans is unknown.

There are also safety considerations. In people with joint complaints, a serious joint infection caused by a difficult-to-treat bacterium has been described following repeated steam therapy; this involved herbal steam therapy, not a classic sauna bath. People with a rare hypersensitivity to their own sweat components can develop skin rashes and severe itching from sauna bathing. For everyone else these risks appear small, but it is worth being aware of them.