The mind is
far more powerful than the body--to the point of being able to control the body's reactions to stress--within the scope of an already-homeostatic state of health, which zeroh likely is in possession of at the present time. Physical health deficits preclude a desire to mentally challenge one's self, according to the principles of Maslow's heirarchy.
If the mind is seeking health, the body will not become open to diseases by such a thing as alternating hot and cold water in the shower. Passing out because of extreme hot water is probably the only problem. If your shower drain is patent, you will wake up with a headache.
If not, get some Drano...
Otherwise, what's to worry about? The stress from worrying about such things plants far more seeds for potential disease than a little 'harsh' conditioning in favor of growth--but lending one's own stress to another in well-meaning health-related advice doesn't truly help, either.