Tuan, our guide, chartered a clunky Russian jeep to take us a few kilometres out of Sapa to the starting point of the trek. Despite his earlier reassurances, the trails down to Ban Ho village were treacherous. Going down was not so bad at first, once I had become accustomed to falling over and getting covered in mud. But when the descent got steeper it became as slippery as a ski slope, and I had the same problem as any novice skier: there was no way to stop a slide once it had started. Soon the inevitable happened, and after yet another slip I found myself skidding and bouncing uncontrollably down the hill on my arse. It actually felt quite good - it took a lot less energy than walking, and I didn't have to worry about falling over any more. I was getting muddy, but I was muddy already. Just occasionally I needed to put a foot to the right or left to keep me on course, but after a while I started to sit back and enjoy the ride. The sky was misty and grey, but I could still see across the valley to the terraced paddy-fields on the far side, and down to the river below. The river was getting closer. More accurately, a ledge was getting closer, beyond which was a drop into the river. I half-thought about letting myself go over the edge - at least the water would clean off some of the mud - but came to my senses in time and by grabbing hold of bushes and clumps of grass managed to stop the slide just short of the ledge. I walked the rest of the way down to the river without a single slip, and felt sure that the worst must be over.

No such luck. After crossing the bridge, the trail up the other side of the valley was steep and muddy. Five steps forwards, four steps backwards. I must have looked like a cartoon character whose legs spin around in the air for an unphysically long time before they achieve any forward motion. But I wasn't finding it funny; I was sure that the next slip would bring serious injury. Elles and Tuan were already several metres above me, and I had no idea of how they'd got there. I stood there looking at the trail, and there seemed to be no foothold on the shiny mud surface. I just wanted a helicopter to come and pluck me off the side of the mountain.

Stop, count to 10, keep going. With a few more slips and a lot of foul language I made it to the top, where a road with a much more gentle gradient lead down to Ban Ho village. My legs had turned to jelly, and I could feel something moving down my left arm - I assumed that it was a bead of sweat - but I was just thankful to be on relatively solid ground. I've never been that scared on a trek before. Not in Nepal, Thailand, or even that time when I got lost in the English Lake District and walked for two hours in completely the wrong direction, into the middle of nowhere.

I regained some sort of composure during the walk to the village, and gradually began to feel a slight pain in my left arm. There was also a suspicious bloodstain on my long-sleeved shirt, and I started to suspect that whatever I'd felt earlier on my arm had not been a bead of sweat after all. When we reached Ban Ho I took of my shirt to reveal the shredded remains of the leech that had crawled inside my clothing and sucked my blood. Bits of the little critter were stuck all over my arm, and the sleeve of my t-shirt (which I was wearing underneath the shirt) was one huge bloodstain which soon dried and started to smell like a butcher's shop. It looked like I'd been shot, but after cleaning myself up the wound was just a single small bite mark, the calling-card of a one-toothed vampire. Luckily the accommodation at Ban Ho was comfortable, and we ate dinner looking out over the lights of the village. A large crowd had gathered in the school, one of the only places with a television, to watch the football semi-final between Vietnam and Thailand. Every so often an aborted cheer would indicate a Vietnamese near-miss. Vietnam lost 1-2.

We passed through many small villages during the two days of the trek, and stopped in some of the houses - there seemed to be a few that had made it onto the tourist route, and were standard rest-stops for trekking groups. While it was interesting to have even this brief glimpse into the lives of the villagers, I was uneasy about the routine that these visits seemed to follow: we would arrive at a house and be introduced by Tuan, sit down around the stove in the kitchen, and then one by one the children would emerge with armfuls of bracelets, blankets, hats and scarves to sell to us. It was the same procedure in every house we visited. Sometimes we bought something, sometimes we just gave a small donation to avoid cluttering up our packs even more. We tried to be careful and only give small amounts - large gifts, especially to individuals, can be counter-productive, since they encourage a begging culture and can create dangerous financial imbalances within a village. Unfortunately, many of the villagers know the emotional power that cute little kids can have over rich tourists who retain the particularly western attitude that simply giving people lots of money will make them happy.

Maybe it's a fair exchange - the tourists get their photos and a few souvenirs, the villagers get a little money. But there seems to be a lack of any real connection at a human, emotional level, little in the way of cultural understanding or sensitivity on either side. It's purely a business transaction: the tourists are there to buy things, and the villagers are there to sell. The whole thing started to feel like an open air museum, where I paid my entrance fee and was led around the exhibits. It felt stage-managed and false, in complete contrast to my experiences in the villages around Mae Hong Son in Thailand, where no-one - no matter how poor the village - seemed interested in my money. The difference, of course, is that while in Thailand most of the villages I passed through saw very few westerners, the villages around Sapa are firmly on the tourist trail, and their culture has adapted appropriately.

Back in the hotel in Sapa, with my blood- and mud-soaked clothes at the laundry, I looked out of the window and watched another coach-load of tourists pull up outside. They were immediately set upon by a swarm of blanket-sellers. There is an argument that if tourists had never come to Sapa, then many of the surrounding hill-tribes would have vanished years ago, integrated into Vietnamese society by the government. As it is, the revenue from tourism provides a powerful incentive for the government to leave the tribes alone. There is certainly some truth in this, and various NGOs and aid organisations have reported on the benefits that tourism can bring to poor communities. But still, when it was time for me to leave Sapa, I felt that it was not right for me to be there.

As the minibus out of Sapa descended towards Lao Cai, the fog grew thinner and the temperature rose. We were leaving the mountains. It was all too much for one man, who projectile-vomited all over the woman in front of him. He put his hands up to his mouth, but this only had the effect of diverting the spew sideways and even backwards, so that the people on either side of him also got splattered. I only escaped because I was sitting directly behind him and was shielded from the blast. On the night train back to Hanoi I was woken in the middle of the night by a knocking on the door. It was the conductor, and I wished he could choose a more sociable time to check tickets - but it turned out that he just wanted to lie down on one of the spare bunks. At least he didn't snore.