system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are most salt water fish able to be eaten raw/uncooked while freshwater fish cannot be?
gpt
Humans are freshwater animals with about 5% salinity. This means the parasites/bacteria in freshwater fish can live and thrive in us as well, and make us sick.

The bacteria/parasites that thrive in salt water animals (~20% salinity) don't do well inside of us. They *can* survive, and you can still get sick from saltwater fish bacteria, but it's less likely.

For reference, all "raw" seafood you get has been frozen first to kill parasites (and *all* fish has parasites, especially farm raised fish). In the US, it's an FDA requirement, and it's a traditional part of sushi preparation. Without that, whether it's salt or fresh water, you're very likely to get something bad from raw fish.