Ich is almost always present in every aquarium, and they are brought in on the "healthy" fish you purchase at the fish store.
A fish can carry ich without you knowing it. The ich disease as we know it is actually an advanced infection. Ich initially inhabits the soft, relatively unprotected tissues of the gills first.
Advanced stages of ich infestations are caused by stress. Stress causes the immune system to funtion at a reduced level. As a result, the slime coating (a protective layer) is breaking down, allowing the ich parasite to attach itself to the fish. Ich can further stress the fish, and this will eventually lead to the death of the fish if left untreated.
In the past, ich has resulted in massive losses of fish in commercial catfish farms.
In my opinion, aquarists pay too much attention to ich. They should pay more attention to the fish. If you can keep the health of the fish at optimal levels with regular water changes, proper diet, reduction of external stressing factors (temperature, filtration, light cycles, etc), and general overall cleanliness, ich should never be a problem for you.
However, about every aquarist will see ich rear its ugly head at some point in their day, and ich must be dealt with swiftly and effectively. I would consider this the "ich contigency plan." Keep in mind, ich should not be considered as a normal disease. A diseased fish is a diseased fish; it represents the result of an unhealthy enviornment in one way or another.
To avoid sounding like the last statement is a cop-out, I will elaborate further. One must understand that fish are extremely delicate relative to the way the average person would percieve an animal. For example, you take the so called "intelligent" human being, and give them fish, but they die. You see the same go into Wal-Mart and buy diseased fish and walk out the door with 15 feeder goldfish in one bag to put in the 5 gallon they have at the house.
Fish are always on alert. You cannot stop natural instinct from flowing through their blood. Their life is pretty simple: must eat, must avoid predators, and must breed. Even your presence to fish can create a stressful situation. After a while, especially after they associate your presence to food, they will warm up to you. I hope this give you at least a mild conception of how fish are affected by stress and vice versa.