We all know about placebos, right? From the Latin placēbō, "I shall please"? When you take a placebo, you feel better because you think you're going to feel better. Of course, it's just your imagination, right? You don't really feel better; it's all a fake, right?
Not so much. A double-blind experiment on Parkinson's patients with fluorine-18 labelled L-dopa vs. placebos showed that the placebos were effective at the specific goal of causing the brain to produce dopamine. The patients didn't just "feel better", their brains actually performed the same biochemistry that is stimulated by L-dopa. Think about that. Think very carefully. These people simply thought they were getting L-dopa; they were not trained in using the mind's power over the body. What if they were?
Yeah, yeah, we've all heard that woo-woo stuff before, right? Okay, never mind. Just remember that the "Placebo Effect" is real. Officially real, as in, no drug trial that fails to take it into account is considered valid by... well, anyone. Now think very carefully about the implications of this established fact.
If you believe a pill is going to make you better, it will make you better. Perhaps not as much better as a pill that has a direct effect on your biochemistry, but genuinely better. If this is true, then the opposite might be expected to be true as well: if you believe a pill is going to make you sick, it will make you sick.
And not just pills. Also innumerable environmental poisons, radiation, even stuff that used to be considered harmless but has now been shown (or claimed, and believed) to be detrimental to health at some concentration. If you believe it will make you sick, it will make you sick.
This is called the "Nocebo Effect" (from the Latin nocēbō, "I shall harm", from noceō, "I harm"). It has also been called the "Antiplacebo Effect" or the "Negative Placebo Effect", but that seems weaker to me. I want to make a strong point about this response, not just a codicil to the literature on the Placebo Effect.
Today there is an epidemic of allergies and autoimmune disorders, especially among young people who have spent their whole lives being told of all the ubiquitous poisons in their environment, and have never been told that the human body is incredibly resilient and robust. Some of this may be genuine reactions to actual pollutants and unhealthy foods, but it must also be due in part to the expectation that everything we eat or drink or breathe is poisoning us.
Notice that I said, "...in part..."; I didn't say that environmental poisons don't exist, or that all maladies are caused by bad attitudes. But what you believe is a significant contributing factor to what you experience. This fact is no longer the property of New Age mystics. It is real science now. Unfortunately, we seem to have no idea what to do about it except to excoriate anyone who disputes the notion that we are all helpless victims with no agency in our own lives.