Question 1026779
Your first question:  We say that the event of having stomach ulcer is independent of having stress.  One can have stomach ulcer without being under stress, and being under stress doesn't mean you are going to have stomach ulcer (although sometimes it could be one of the symptoms.)  One could occur without the other, or both could occur at the same time.

Your second question:  Independence is the non-significance of conditioning in probability.  Given you are under stress, the probability of HAVING (not aggravating) stomach ulcer is the same probability as having stomach ulcer without stress.  Meaning, the condition of stress is just immaterial in the occurrence of stomach ulcer.
Mutual exclusivity is concerned with the possibility of having two events happening AT THE SAME TIME.  Obviously, stress and stomach ulcers are not mutually exclusive events.