I think you might have gotten these answers on your other thread, but to clear things up incase you still have questions,
A, They get in through adding new fish that had them, or through live food.
B, they live in a fish, feeding off of it internally. Much like worms you find in dogs, cats, horses, etc.
C, prevention is the best thing. ALWAYS quarantine new fish. Medicated food and seperating any fish that looks like it has parasites is all you can do.
D, yes they do spread when their host dies.
E, Yes, it usually kills the fish.