What You Don't Know Can Slight You

A rodent given a saccharine drink and then an immunosupressive drug will, ceterus paribus, suppress its immune system characteristically of that drug when later given the drink. Though saccharine is commonly unhealthy, this is a foundational study of P(sycho)N(euro)I(mmunology).¹

You don't know what'lls happening so you should act carefully. For example, repeated exposure to a stress hormone elevation can increase its inhibition of immune function²; but you don't know how such entrainment will affect its other stress stimuli in situ.

¹http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=2537692
²http://www.sciencedirect.com/science/article/pii/016557289290014C