Without stress, most fish are able to fend off parasites. The parasites are always there, just in numbers that do not cause issues. But let stress enter the picture like high ammonia, chlorine, high nitrites, very high nitrates, large pH shifts, major water temperature shifts, or predators, and the fish can weaken and the parasites take advantage of the situation. Not all fish will have the same level of stress so not all will succumb to the parasites. Now, before doing any treatments for parasites, it is always best to rule out water issues. We like to see actual numbers for ammonia, nitrite, pH, KH and temperature. You can in addition, look at the temperature shift during a water change, and if possible to reduce temperature differences, draw a bucket of water right after a water change and allow it to sit next to the aquarium to attain a similar temperature for the next change. If you have ammonia, treat with a dechlorinator that also binds ammonia, like Prime, Safe, or Cloram-X.
Without the water being pristine, (ammonia=0, nitrites=0, pH stable morning and evening, temperature stable) any treatments for parasites will do little to stop or prevent infections and if the fish has infections of any type, then the healing will be slower.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard