What causes a person to complain and be preoccupied with minor illnesses such as nausea? How should the caretaker respond?
First, the patient needs to have a proper medical exam to rule-out an underlying medical conditions (such as increased gastric acidity or reflux). Once this is ruled out or treated, then addressing the excessive worry is important.
Preoccupation can take the form of a minor worry that passes to a persistent intrusive thought that causes significant distress or impairment. In the latter circumstance, there may be an underlying unrecognized psychiatric disorder - for example, obsessive compulsive disorder or depression - which if treated may well lead to a resolution of the illness preoccupation.
A psychiatric consultation would be wise. The patient (and the caretaker) may well benefit from reading a book such as "It's Not all in your head" by Asmundson or Carla Cantor's book "Phantom Illness. Both are excellent books that describe the psychological mis-steps that lead to illness preoccupation.