From my understanding fish with good immune systems will carry ich, velvet, flukes etc in small not noticeable amounts ( meaning you won’t even get flashing or spots) and aslong as the fish stays healthy it can keep those small number of parasites for essentially forever? Another way to ask the same question would be if you were to put a fish through observation and that fish was previously exposed to ich, velvet, etc even if that fish made it through a observation without signs of a outbreak because it’s immune system is working and fighting the parasites off it would still technically be carrying the disease into your tank and cause a possible outbreak right?