Ich is quite common. It is carried by a lot of pet store fish, and triggered by stress. It is highly contagious, so don't be surprised if all the fish in your tank get it.

Make sure you follow the package directions exactly when treating it. Some medications are much more effective than others, but also harsher. Some ich medications can't be used on sensitive fish (like tetras), scaleless fish (like some catfish), and will kill invertebrates (like snails). Of couse, the stronger meds are often the more effective, so it's a toss-up if you have a bad case.

Ich is easily cured and rarely fatal, unless you let it go too long, or the fish is already weak, ill or highly stressed.