 In 1975 in the United States , a serious outbreak of oyster @-@ vectored disease was caused by Vibrio vulnificus . Although the number of victims was low , the mortality rate was high at 50 % . About 10 cases have occurred annually since then and further research needs to be done to establish the epidemiology of the infections . The cases peak in mid @-@ summer and autumn with no cases being reported in mid winter so there may be a link between the temperature at which the oysters are held between harvesting and consumption . In 1978 , an oyster @-@ associated gastrointestinal infection affecting more than 2 @,@ 000 people occurred in Australia . The causative agent was found to be the Norwalk virus and the epidemic caused major economic difficulties to the oyster farming industry in the country . In 1988 , an outbreak of hepatitis A associated with the consumption of inadequately cooked clams ( Anadara subcrenata ) took place in the Shanghai area of China . An estimated 290 @,@ 000 people were infected and there were 47 deaths . 
