Any marine hobby book will have info on fish diseases, and there are other hobby books that are totally deadicated to the disease subject. Check out Amazon.com for books and book details. Additionally, there are many hobby sites on the WWW that can give you details on fish disease, epecially ick, as it is the most well-known.
To get you started, let me give you some info in a nutshell....
Saltwater ich are found in nature and normal, healthy fish commonly carry a few of these parasites when collected. Left in the ocean, fish will experience no real ill effects from ick unless some other stressor is present---for instance an injury to the fish. Once captured, however, stressors to fish are extreme [please see TerryB’s series on stress in the Fish Forum] and the fish become vulnerable to parasitic infestation. Thus, a parasite that causes little trouble in the wild, becomes a menace in the captive environment.
Most find that there is no effective treatment for ick as long as the parasites remain embedded in the fish; some, however, purport that FW dips are an effective initial treatment of ick [while the parasites are embedded in the host fish] to be followed by other, longer term conventional treatments, such as hyposalinity or copper treatment. The standard treatment for ick is hyposalinity or use of copper. My preference is hyposalinity.
Ick has a life cycle of approx. 23 days during which time the parasite undergoes 3 stages to complete its life cycle. In the tomite [free-swimming] stage, the parasite is infectious to fish. During this stage, the tomite’s goal is to find a host fish, or die trying. After they attach to the gills or body of a fish, they develop into the second stage, the parasitic trophont. During this stage they burrow into the fish, feeding on it’s tissues, which can cause considerable damage and even result in a secondary bacterial infection on the infected fish. Once well fed the trophonts stop feeding and develop cystic coverings. This becomes the inactive tomont stage and during this final stage the cysts may stay trapped in the mucus of the fish, or fall off and sit on the bottom of the aquarium. Within 6 to 10 days hundreds of new tomites emerge looking for fish hosts and the cycle begins all over again, and again and again until something is done about it. The only time this parasite is vulnerable is during their free swimming stage [unless you are a hobbyist who strongly believes in FW dips & that ick are also effected by dips]. The standard treatment for Ick is copper sulfate and hyposalinity. Only in the free-swimming stage are these 2 treatments effective.