Stony corals and soft corals are subject to disease in the same way as other organisms. This may not have been obvious in the past but is becoming increasingly apparent in the twenty-first century. The ill health is the result of the corals being subjected to increasing amounts of stress as the physical environment in which they live becomes less suited to their needs.
Corals live within a precise range of environmental conditions including water temperature, salinity and water quality. Variations outside the normal range of these parameters may make the corals less able to grow and reproduce successfully. Of themselves these variations may be insufficient to kill the corals, but they make them more susceptible to disease organisms. The main factor that causes stress to the corals is climate change, with an increase in sea temperatures, particularly affecting shallow-water corals in the tropics. One of the consequences of heat stress is that the coral expels its zooxanthellae and becomes bleached. The rise in sea temperature is also expected to increase the frequency and severity of tropical storms; these adversely affect corals through mechanical damage to reefs, through increased wave action, and through the stirring up and re-deposition of sediment. Other stress factors include increased pollution, increased ultraviolet radiation, and a reduction in the aragonite saturation of surface seawater that is connected with ocean acidification. Although stressed corals are more susceptible to coral diseases, it is infectious organisms that actually cause these diseases. Pathogens so far identified include bacteria, fungi and protozoans.