I have severe depression and no one in my house has developed depression on account of that...My whole family has various mental issues to varying degrees as far as I can tell though I am the only one who has major difficulties functioning and can't hold a job and probably the only one that's been in the psych ward not entirely sure on that though...my brother doesn't have a job but not because he can't work or interact well with people he just hasn't found one yet and is still looking and applying. My 12 year old brother does not work either obviously so it has yet to be seen how functional he'll end up.

So I don't know I think its more of a shared genetic thing, not so sure totally unrelated people would end up developing mental illnesses from each other...though if someone is around someone who is really depressed and can't mask it at all they might start feeling depressed, but its not the same as getting depression...chances are in that situation the person could not be diagnosed with a depressive disorder.

Also I don't think collective hysteria is really an example of mass mental illness, people without mental illness can get sucked into these kinds of things...after all mental illnesses all have to do with normal human traits/behaviors that get to the point they negatively interfere with someones life. So yes paranoia is a symptom of some mental illnesses, doesn't mean people without mental illness can't get paranoid...paranoia and propaganda was what the nazis used to get people to support their violating the rights of many people. People can feel depressed without having depression, its natural to feel depressed about things. Though if one is in a situation causing them to feel depressed for long enough they could develop the disorder since the brain can go through changes based on environment even if there is not a specific genetic factor...at least I think that is possible not entirely sure.