A disease is classified as any impairment of vital body functions, systems, or organs. Diseases are a natural aspect of coral populations and are one mechanism by which population numbers are kept in check. Disease involves an interaction between the coral host, a pathogen, and the reef environment.
Coral diseases can be caused by bacteria, viruses, protozoa, or fungi, and can cause significant changes in reproduction rates, growth rates, community structure, species diversity, and abundance of reef-associated organisms. ref
Coral diseases have increased in frequency, intensity, and geographic range over the last several decades. Scientists do not know if the emergence of disease outbreaks is due to the introduction of novel pathogens or due to changes in existing pathogens that may be caused by deteriorating environmental impacts and/or reduced host resistance. Scientists do know, however, that coral diseases combined with hurricane damage, coral bleaching, and other stressors are resulting in widespread coral mortality.
Many coral diseases have not been thoroughly characterized and the causative agents of many diseases remain unknown. This is still an area of active research. Understanding the causes (including environmental drivers and modes of transmission) and impacts of coral diseases is essential to help managers understand how current reef management practices and human impacts affect the spread and severity of diseases, and make informed management decisions. The global increase in coral diseases presents serious concerns for coral reef managers and stewards of tropical marine ecosystems.