This is from the article I linked to....
When the parasite is visible to the naked eye, it is a nearly fully developed trophont which has burrowed under the fish’s mucus coating where it is protected from chemicals (medication). It has likely been feeding on the body fluid of the fish for several days and has swelled to many times its original size. At common aquarium temperatures of 75 to 80ºF this feeding stage lasts only a few days, at which point the fully developed cyst drops off the fish as a tomont.
The tomont may swim for several hours before settling on and attaching to the substrate, a plant, or some other surface. During that time it is susceptible to chemicals and medication will be effective. Once attached, it begins its reproductive stage. It encysts and begins rapidly dividing. At this point, it is again immune to chemicals. Within a few days, hundreds of new organisms burst from the cyst, sprout cilia and start swimming in search of a host.
These are now referred to as thermonts or swarmers, and they must find a host within a few days or they will die. (For this reason, we know that even an aquarium heavily infested with Ich would be “clean” and safe for new fish after only a week or two without fish in the tank.) Medication is effective at this stage. Once the thermont attaches to a host and burrows in, it is referred to as a trophont and the cycle begins again. Unfortunately, with each cycle the number of organisms in the tank increases dramatically.