Before reading this post, it would be advisable to read “A Closer Look at Ick - Part 1 (The Life Cycle)”. Several of the terms and concepts mentioned there will be referred to in this post.
Ick (also known as ich) is short for ichthyophthiriasis and is caused by a parasite called Ichthyophthirius multifiliis. This organism is a protozoan, which is a general term for single-celled animals. Although it is only made up of a single cell, it has many features that make it extremely good at what is does. In fact, ick is so successful it has become the most common disease of captive goldfish, and is responsible for more goldfish deaths than any other disease.