How does feces and a parasite get on so much lettuce? I'm assuming it's someone with the parasite shit in the field and it got on some lettuce, but how can like one persons shit get spread to 23,000 people's lettuce? Or is it something else like they are fertilizing the fields with human feces or something?

I think a lot of small scale organizations don't use human feces for food because of pharma contamination. Not particularly surprised if only .001% of food is done correctly in the US though.