Hmm very interesting question.

I am not quiet sure- i think a lot of research would need to be done to actually answer this,

and even then- as time goes on perhaps more information could be found on Mass Hallucinations

After mass hysteria was not known when it was happening until history was studied of places of where it occurred.

I am sorry I am not giving an answer on this- but seeing this is your one post, i wanted to let you know that some one read it

Be well, I am going to look some of this up.

Here is one site that has your same question:
http://lesswrong.com/lw/2sr/are_mass..._a_real_thing/