By their evil thoughts people arouse certain emotions which cannot be
satisfied and which therefore have to be repressed. The effect of
repression is either chronic ill-health which no medical skill can ever
cure, or organic disease that is beyond the art of man to heal. The
reader must not imagine, however, that everyone who suffers in this
way has been guilty of evil thinking, but the practice is certainly one of
the ways by which some people can, and do, literally destroy

1. Does ", or" above means "that is"? But I don't think so because there is first "either" and then "or", nevertheless sometimes I'm not sure whether ", or" means "that is" or just presents another alternative.

2. Does "must not" mean that the reader "shouldn't" imagine?
1-- It offers an alternative: either chronic ill health or disease.

2-- Right.
Thanks for the fast reply!