Recommendation: Do your research.
From Luang Prabang, I spent $13 to take an 8-hour bus ride to Luang Namtha because I heard there was great trekking. When I arrived in the tiny town, I walked around in search for a tour company that could take me trekking. There weren't many good options, but I eventually found a company that seemed OK, so I signed up for a 3-day, 2-night trek leaving the next day. From that point, things turn a rough turn... I went get some dinner at a market.
As I ate a papaya salad, I remember thinking it tasted extra fishy, but attributed it to them using too much fish sauce. The next morning, I woke up feeling a little sick, but not bad enough to stop me from going on the trek. I walked over to the meeting point where the group piled into a Tuk-Tuk and headed to a market to get food. Stop reading here if you're a very visual person!
The market was filled with live rats and other rodents in cages, dead puppies spread across tables, and I'll just stop there. I didn't last long before the urge to vomit hit me. I ran out of the market, threw up on the side of the road, and almost fainted. I think the smell and sights in the market induced whatever minor stomach illness I had into full effect. They drove me back to my hotel where I spent the next week in the worst shape I've ever been. The nearest hospital was 6 hours driving. I opted to try sleeping it off. Each day I got a little better, so I skipped the visit to the rural hospital. When I was finally strong enough, I got on a bus back to Luang Prabang and got myself out of Laos.
Reflecting on the experience, I should've listened to my gut. My first mistake was showing up to this very rural town with no plan. The trekking agency seemed sketchy when I signed up, but I signed up anyways. Second, I should have stopped eating the fishy papaya salad. I can't say 100% if it was the papaya salad or the market that put me over the edge. In hindsight, I think I was meant to get sick to avoid whatever disasters were likely to unfold with the sketchy tour group.
On a positive note, over the remainder of my travels, I never got sick again!