The thing is until you make friends with someone they are a stranger to you, or if you are super optimistic or naive you could write "There is no such thing as a stranger, they are only friends you have not meet".
But I would warn people to keep their heads switched on to make sure that they do not fall victim to the on line version of Myra Hindley.
I have thought about the some of the possible different random messages which you could get and what the best thing to do in each case is.
1. A stranger on WP writes a PM to you after hearing that you are ill, injured or in trouble offering you their best wishes and hoping that things get better.
Reply to this it is a kind hearted person.
2. A person telling you about something you have never posted.
Check what you have posted, then send them a polite PM telling them that you think that they have mistaken you for someone else. It happened to me once when someone got the authors in a thread mixed up.
3. Something rude, sexual or threatening, eg "I want to $%£& you and £$%&* etc"
Do not reply, instead tell a mod
4. Someone trying to sell you something, eg "Do you want to buy Viagra, timeshare, new hard disk, stocks and shares etc"
Do not reply, instead tell a mod
5. A stranger on WP writes to you about some topic you have posted on, they might be offering you an insight or information.
Do reply, also they might become your friend
6. A stranger who has asked you to get involved in a controversial matter, who is trying to recruit you to fight his battle.
Do not reply, instead tell a mod
7. A stranger who asks you to get involved in some commercial activity which looks too good to be true. That may well be some new version of the 419 fraud.
Do not reply, instead tell a mod
There are more types of messages from strangers which I can not think of right now. My advice is to always think before you reply or do anything.
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Health is a state of physical, mental and social wellbeing and not merely the absence of disease or infirmity

I am not a jigsaw, I am a free man !
Diagnosed under the DSM5 rules with autism spectrum disorder, under DSM4 psychologist said would have been AS (299.80) but I suspect that I am somewhere between 299.80 and 299.00 (Autism) under DSM4.