In a personal experience I had a tank get velvet, all died but a puffer, then even got a new tank, cycled, finally added the puffer, then some new fish. Velvet again! I called it a trojan fish. It didn't die from it, but it must have carried it. Another theory (off topic) is that a puffer can carry parasites in it that are spread when it puffs out opening 'things' in its body and releasing them...