Biotic diseases are instigated by microorganisms including bacteria, fungi, protozoans and possibly viruses. Abiotic stresses occur in response to physical or chemical stresses, such as increased sea surface temperatures and UV radiation, anthropogenic input of nutrients, sediment and other pollutants. Abiotic stresses may exacerbate the role of microbes in disease processes. Thus, the recent emergence of disease in corals is believed to be related to deteriorating water quality associated with increased land-derived sediment, pollutants and nutrients, and increased sea water temperatures. These changes may favor the proliferation and colonization of microbes, and they may reduce the ability of a coral to defend itself against biotic and abiotic stresses.
Most diseases appear to be a response to multiple factors.
Although coral diseases are damaging reefs around the world, they have been documented most frequently in the western Atlantic. In the 1990s, there has been an unprecedented increase in disease of scleractinian corals in the Caribbean, and over 15 syndromes are now reported to affect corals. Coral diseases are also having an impact on other important reef-building corals worldwide, but assessments in regions other than the western Atlantic are limited to a few reefs. These efforts should provide needed information on the natural level of coral diseases to determine whether disease occurrence is being exacerbated by controllable stresses and should assist in the development of strategies to mitigate coral diseases. In order to assist regional monitoring organizations in collecting information on coral diseases, NMFS has produced bilingual underwater coral disease and coral predator identification cards .