Fish which survive ich acquire an immunity against it. How kibf thia lasts is uncertain and different species will vary in their response. If you are interested in the science on this topic, go to
Google Scholar and enter "acquired immunity to Ichthyophthirius multifiliis" in the search box.
Ich is the bane of both the ornamental and the food fish industries and has been for years. There is extensive research into finding a vaccine for Ich.
Here is a pretty interesting paper which discusses a ton of treatments tried for Ich. You will not believe how many things have been tried and administered in various ways.
Picon-Camacho SM, Marcos-Lopez M, Bron J & Shinn A (2012) An assessment of the use of drug and non-drug interventions in the treatment of Ichthyophthirius multifiliis Fouquet, 1876, a protozoan parasite of freshwater fish, Parasitology, 139 (2), pp. 149-190.
http://dspace.stir.ac.uk/bitstream/...macho et al Parasitology Ich chemo review.pdf