Well, you can if there is a mental connection to the immune system. E.g. attitudes toward life and toward disease seems to affect outcomes in cancer patients. And placebos can affect that.
That's true, but I'm not sure such a link has ever been established. Is there any study that has found measurable variance in cancer or infection outcomes based on differences in mental outlook?
In short, yes! Mental outlook has a big effect on outcomes. It won’t cure you, but it will improve your physical outcomes.
This is the meta analysis they refer to:
Rozanski A, Bavishi C, Kubzansky LD, Cohen R. Association of optimism with cardiovascular events and all-cause mortality: a systematic review and meta-analysis. JAMA Netw Open. 2019;2(9):e1912200. doi:10.1001/jamanetworkopen.2019.12200