WTF causes ick? Just bad water conditions?
Ich is most often brought into the tank or pond on new fish or plants (not if they are quarantined!). However, it is also believed that some survivors of an Ich infection can become latent carriers, with the parasites forming a latent stage at protected sites such as the base of fins or the gills.
Subsequent stress or poor conditions can awaken white spot to re-infect either its host or other fish. This certainly seems to be the case with koi, when often small numbers of trophonts are often found alongside severe fluke, Trichodina or Costia infestations, even in ponds which have not had any new introductions.