Interesting and current example of how thoughts/emotions can affect your health: after white powder was found on a post terminal in vestfold, fear spread and 40 people needed medical treatment, 14 was quarantined. The white powder was later confirmed to be harmless flour. This according to ap.no.
This is called the nocebo effect, which is the opposite of the placebo effect. One experience a worsening of health because this is expected.