What are the odds the average first worlder is free of intestinal parasites? Although we have a very low probability of catching them on a given day, we also take no measures to remove them by e.g. taking a dose of ivermectin once a year (as they do elsewhere). It seems likely to me that I would have some kind of worms just based on this probabilistic argument. But like anything with probabilities and counterfactuals it’s hard to be confident without good data first