The parasites that cause
ICH are usually present in small quantities in all water, including the water that comes from your tap. Normally, they're harmless, but when fish are stressed and their immune systems are weaker, the parasite becomes opportunistic and infects the fish. After a few days the parasite erupts, and sprays more spores into the tank, increasing the chance of other fish getting infected. Increasing the temperature speeds up the cycle, and also kills off the spores before they can re-infect the fish. They're free-floating, and only attach to live hosts.