This is a very difficult topic to learn about, as for the most part, not enough scientific study has been completed yet. Coral disease research really did not get a lot of focus until the mid 1970s with many of the common diseases only becoming properly identified in the early 1990s. With the increasing levels of pollutants in the waters of the coral reefs, combined with the effects of global warming, more and more diseases have be developing, along with the frequency they are being found in among the ocean’s coral reefs.
This is (IMO) a very serious threat to the long-term health of our coral reefs today. For example, White band disease has had a devastating impact in the coral reefs in the Florida Keys (killing ~95% of all Acropora SPS corals), and in the Caribbean (infecting ~50% of the shallow water, mostly Elkhorn SPS corals within the first five years after this disease was discovered there). During the past 10 years, the Caribbean coral reefs are being affected more and more each year. New diseases have even started to surface that affect coralline algae (Coralline Lethal Algae Disease) which could also have a significant impact on health of our coral reefs.