Since the early 1980s, diseases have overwhelmed oyster populations in the Chesapeake Bay, causing losses of up to 90 percent of stocks in some areas. These heavy mortalities have been linked to the spread and intensification of two parasites: dermo (Perkinsus marinus) and MSX (Haplosporidium nelsoni). Though deadly to the oysters, neither affects humans, whether the oysters are eaten raw or cooked.
Dermo is a disease caused by a single-cell organism (protozoan) that infects oysters, eventually reaching such high numbers within the oyster host that it can no longer maintain its physiological functions, and dies. The exact mechanisms by which the parasite kills the host are not understood. Dermo tends to have its largest impact on oysters about the time they reach market size.
MSX disease is also caused by a protozoan. It is more virulent than dermo. And, as far as is known, it also kills by reproducing to vast numbers within oyster tissues and overcoming the oyster's ability to maintain its functions. MSX does more damage to young, or small, oysters than dermo.
Source: Maryland Department of Natural Resources