Most people think that if a fish dies, it's going to foul the water and it's going to kill the other fish. Whatever killed that fish is now in the water, and I now have to change all the water. That's a misnomer. You don't want to do that. You want to remove the fish from the tank if you can find it, but unfortunately with a lot of large aquariums, if a fish dies, it's stuck behind a rock and it's very hard to find. There's nothing wrong with leaving it in there. It's just going to produce a little bit of nutrients that your filter will then have to remove. But, if you have a strong filter and you keep it clean, it should easily be able to handle the waste produced by the one fish that has died.
Most living parasites and diseases a require a living host. Once that fish dies, those parasites and diseases have already moved on to the water column or to other fish. So, it's already in your system. You don't have to worry about removing the fish to prevent this disease from spreading. It's already probably already affected your other fish. You just didn't realize it yet. Or, the other fish are stronger, their immune system is strong enough to fight off whatever killed that fish, if it wasn't just water chemistry or stress alone.
So, as a rule of thumb, always remove your fish, but you do not have to remove the fish or the water from the aquarium when a fish dies. It's probably good practice to investigate. Any time a fish does die, it does tax the system a little bit more, but you certainly don't have to remove all the water and remove the fish when a fish dies.