Watchtower 15th March 1953 page 191.
''There is no reason to think that they [demons] cannot possess persons today.
This is not to say that ALL those mentally sick are so possessed, for they are not.
Mentally sick persons, however, in their weakened state, may present more vulnerable targets for their assaults and invasions.
SOME show evidence of demon possession, whereas others show evidences of merely mental sickness or nervous breakdown.'' (Emphasis mine)
I for one would not want to work with people with mental illness or depression just in case they were possessed by some of those pesky spirits.