Cryptocaryon irritans, aka Ich...is a parasitical infestation. As a parasite...Ich is an opportunistic type protozoa...once a fish's immune system is lowered...the parasite will attach to the new host.
The immune system is more often then not, lowered by stress. Many things contribute the the increased stress/lower immune system;
Temp
water quality
additions of new fish
aggressiveness from tank mates
trying to catch them with a net
The list goes on and on. Once Ich is introduced into the tank...it is difficult to be rid of. Ich, left un-treated, and an infestation is obviouse (spots, cloudy eyes, etc), the chances of the particular fish fighting the Ich off are pretty slim, and death usually will occur.
Go the the disease forum here and inquire within about your Ich. They will provide you with sound advice.