Hi,
Just so we can offer some useful advice we will need to know a bit more information.
What size tank do you have and what kind of filtration?
What are the water stats (PH, Ammonia, Nitrite, Nitrate)?
What fish do you have and how many?
How long has the tank been running and was it cycled before adding fish?
Ich normally only effects or kills fish that are already stressed for another reason, normally this is due to por water quality so eliminating that problem with sort the rest out.