think of it this way. theres the flu, and theres tons of strains. some are milder than others, and others like the bird flu kill over 75% of the time.
i've been through 3 major ich waves in my years of the hobby (14 now).
1st wave, ****** wiped out all fishes within a week i had in my tank (foxface, yellow tang, clown, blenny)
2nd wave, cypto killed 70% of my fishes over 2 weeks (new clown and wrasse made it out alive)
3rd wave (most recent) - ich appeared on my flame wrasses and infected my cryptic six line. showed signs for days, went away, came back after temp shock and all 3 made it out alive. none appeared to ever stress during the attack. been 2 months now with no ich