hit the nail on the head shuih!! also other fish can stress them! im not totally sure and one of the more experienced members should correct me if im rong but, white spot could be likened to stress in humans, except for that stress shortens our lives and it tends to kill fish if not combated quiekly, fish always have the virus that causes white spot, but it only becomes a problem once they have become harrased/stressed. wether it be by other fish, transportation from a shop or freind etc or by being ruff and frightening them! also there can be water conditions that lpay a roll in it also!
Cheers Shae