system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How and why is sushi safe to eat?
gpt
Not all the bacteria present in the fish are harmful to humans. Some are, but not all. Even the harmful ones aren't always bad. Our bodies are constantly killing bad cells.  Any parasites or bacteria are generally put to the death quite quickly by our bodies defenses.  

If a fish shows any telltale signs of being unhealthy, then these fish are not used for sushi.  If a fish looks real good, it's cut up and shipped to a restaurant or market.  We have figured out VERY well how to tell if a fish is not healthy.  We also know exactly how to make things even healthier by caring for it properly.

Sushi fish is treated VERY well, and cared for as if a tiny baby.  If a person does not treat the fish properly, it can get contaminated from almost anywere.  If someone touches it without gloves, drops something on it, or lets it sit out too long, the bacteria could start to grow.  If the bacteria is given time to sit and multiply on a tasty piece of fish, it'll start growing into a colony.  One bacteria is easy to kill, but millions are much harder. Sushi is expensive because of the work that humans put into making it perfectly safe to eat.

Often, not always, fish is put into a special freezer that won't harm the fish's taste but will kill the tiny parasites that are harmful for humans.