Welcome to the documentary from the BBC World Service.

I'm health correspondent Matthew Hill,

and for the past year I've been investigating an intoxicating substance

that's killing thousands of people around the world.

I'd met Simone in Cambodia, and we went over to Lowe together.

This is Bethany, who last November had been travelling with friends around Southeast Asia.

We met our other friend, David, in the hostel.

One of the perks of staying at the hostel was that every night between, I think it was

8pm and 10pm, they offered free shots, so free vodka and whiskey.

So we had shots of those and we pulled them into like Sprite.

So we had them as mixed drinks.

Went to bed, woke up, felt a little bit hungover,

didn't feel particularly like anything was wrong at that stage.

Through that day, I just felt really, really fatigued.

We were getting a minivan back to the capital of Vientian.

That's when things kind of started to go a bit downhill.

I hear David say, you need to get off.

Mine's being sick, so I got off the bus, fainted myself, hit my head,

and then we all got back on the bus.

Getting struck down with a mystery illness in the middle of nowhere is everyone's worst nightmare.

But for British backpackers Bethany Clark, Simone White and David Richards, the source of their

sickness was a ticking time bomb.

...