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Marvin,
 
Can't speak clearly about the cause, but a couple of things might help you
feel better when it happens.  My two favorites are Orabase gel applied to
the sores and lysine, an amino acid, taken internally.
 
One reason lysine might work, is that is has been shown effective in
countering viruses such as herpes (like the mouth sore kind), but arginine
encourages the growth of the same sort of virus.  Arginine is high in
gelatin, chocolate, carob, coconut, oats, whole wheat, white flour, peanuts,
soybeans and wheatgerm.  Source: Leon Chaitow's Amino Acids in Therapy.
 
When I started the gf diet, I ate a lot of corn and was also off milk, a
good source of lysine, but corn is known to contain an incomplete protein
because it lacks a good amount of lysine.  I started to get a cold sore ( my
lip was buzzing and a bump was starting to rise) and I took a lysine tablet,
and the sore never erupted.
 
I remember seeing a special on TV that said that either the Mayans or the
Incas survived on a corn diet because they always had bugs in their grain.
(yuck!)
 
FYI, and hopefully helpful,
 
Susan Owens