Kiwi conservationist Pete Bethune had just walked past a pile of leaves when he felt a sudden jab to the back of one of his legs.
Even before looking back, he knew he had just been bitten by one of the deadliest snakes in Central and South America - the Fer De Lance.
Speaking from his hospital bed in Costa Rica, he said he had noticed the bunch of leaves and made sure to be careful around it.
"I just missed him hiding under the leaf. I walked past him and he turned around and hit my calf from behind," he told NewstalkZB.
"When he hit me... I (turned) around and here he is backing up. I knew straight away it was a Fer De Lance and I knew I was in trouble.
"He did put a tonne of poison in me."
The 55-year-old, founder of non-profit organisation Earthrace, was with several crew members when the incident happened yesterday.
They rushed to contact medical staff and make plans to get Bethune to safety for urgent medical help.
Crew members tried to carry Bethune through the jungle - but because the terrain was so steep, he was forced to drag himself for about 2.5km.
"When you get bitten by one of these things, the best action is to lay still."
At times Bethune had to crawl and drag himself down rugged jungle terrain and even through waterfalls to get to safety.
He was later piggybacked by a crew member on the beach, to a vessel that was waiting to get them to the mainland.
Not out of the woods yet
"By the end, I was basically incapacitated," he said.
He said had they not managed to get out before dark - forcing them to spend the night in the jungle - he doubted he would have survived.
Doctors continue to monitor his situation and there may be a need for him to operated on, after his left leg doubled in size overnight.
"I'm worried I'm going to end up with some permanent damage or something like that.
"The poison has reached my groin... and it has gotten progressively worse in the last day.
"The leg is enormous... it looks almost grotesque."
They had seen positive signs in the last hour or so, however, as there had been no further growth.
Asked about the snake after he was bitten, Bethune said it backed away as he and his crew did the same.
It was not harmed and staff only approached it again to get photos of it, as local doctors often wanted proof to know exactly what medication should be given.
Bethune said he was "happy to be alive" and would keep his fingers crossed he would be out of the woods soon.