I’ve come to learn that it is not true that one should not train when they are sick.
I’ve trained a few times now, over the last several years, when I’ve been at least moderately sick.
(Or hurt, or tired.)
I have found that the opposite of ‘mind over matter (or body)’ is true.
It’s ‘body over mind’.
During some of my workouts while sick, I’ve broken PR’s and/or had super solid sessions.
It’s your mind that’s telling you you shouldn’t train.
But once your body gets warmed up, you can feel that your body is actually quite capable.