[TheSoftwareGuy]

>Outcomes >All outcomes were self-reported using web-based surveys.

>Conclusion >A routine (hot-to-) cold shower resulted in a statistical reduction of self-reported sickness absence but not illness days in adults without severe comorbidity.

I'm always a tad skeptical of self-reported data. In this case, seeing as the more objective measure shows a much smaller effect, I am inclined to think this is more a placebo effect than anything else.

Still, a placebo effect that makes you feel better seems like a good thing, so I might just start taking some cold showers now