I have had occasional episodes of this since early childhood and they are almost always when I am over tired or really fatigued. It begins as giddiness and then expands from there if someone 'feeds it' if they too start laughing.
The instant cure is to remove myself from any sort of audience.

Apparently it can be the result of brain lesions or anxiety.

Inappropriate laughter due to brain lesions:
http://neuro.psychiatryonline.org/ar...ticleid=100246

Inappropriate laughter due to personality disorder/anxiety:
http://voices.yahoo.com/can-uncontro...y-4668615.html

With love, Erika