The actor's health scare was revealed through a friend of his -- Scottish broadcaster Gordon Smart -- during an appearance on the BBC's 'The Good, The Bad and The Unexpected'  ... where he told this wild story about him, Jamie and some other pals hitting Portugal last year.

After a night out, Smart says some in their group started to feel really sick the next day -- with GS getting sent to the hospital ... as he felt tingling in his hand and arm. Smart thought he was having a heart attack ... but was later told he'd been infected by a local caterpillar.

Specifically, Smart says he was told he must've come into contact with pine processionary moth caterpillars -- which have poisonous hairs all over their bodies, and which can infect humans. They're not usually deadly for people -- but they can, in fact, kill animals.

Anyway, he says when he got back to the hotel where Jamie was staying -- he saw JD, too, was hooked up to medical equipment ... and noted he'd suffered from the same symptoms.

Eventually, both of their symptoms cleared up -- and in terms of how they might've come into contact with these little buggers ... Smart says they seem to be all over golf courses there, which is where the boys were roaming just before coming down with their illnesses.