a......Some people are afraid of the possibility of ghosts or haunted places. Being afraid will cause someone to be tense and tenseness causes many health problems.
b......Humor and kindness are supposed to be good healers. If a ghost like Casper came along, then the good feeling or laughing at his/her antics could be healing, therefore related to health.
c......For someone overweight, losing weight will make them healthier. If they did not believe in ghosts, then seeing one would cause some to lose a couple of pounds immediately, although they would have to change clothes and possibility of doing one of the healing baths.
Should be more reasons, but this is a start. It really belongs in a health forum list.