White pox disease

White pox disease (also "acroporid serratiosis" and "patchy necrosis"), first noted in 1996 on coral reefs near the Florida keys, is a coral disease affecting Elkhorn coral (Acropora palmata) throughout the Caribbean. It causes irregular white patches or blotches on the coral that result from the loss of coral tissue. These patches distinguish white pox disease from white band disease which produces a distinctive white band where the coral skeleton has been denuded. The blotches caused by this  disease are also clearly differentiated from coral bleaching and scars caused by coral-eating snails. It is very contagious, spreading to nearby coral.

At the locations where white pox disease has been observed, it is estimated to have reduced the living tissue in elkhorn corals by 50–80%. In the Florida Keys National Marine Sanctuary (FKNMS), the losses of living coral are estimated to average around 88%. Elkhorn coral was formerly the dominant shallow water reef-building coral throughout the Caribbean but now is listed as a threatened species, due in part to the disease. Elkhorn coral is the first species of coral to be listed as threatened in the United States.

Serratia marcescens
The pathogen responsible is believed to be Serratia marcescens, a common intestinal bacterium found in humans and other animals. This is the first time it has been linked to the death of coral. The specific source of the bacteria that is killing the coral is currently unknown. As well as being a part of human and animal gut flora, S. marcescens can live in soil and water as a "free living" microbe. Research is needed to find and confirm the exact source(s) of the pathogen, possible sources include sewage treatment plant effluent, marine fish feces and seabird guano.