Fish do not naturally carry a wide variety of pathogens, but some may contaminate the marketed product and these are normally derived from the environment– for example, from pollution of the water in which the fish live, or improper handling after they are removed from the water. Many outbreaks of infectious hepatitis and typhoid have been attributed to shellfish as vehicles. Bivalve molluscs, such as oysters, clams and mussels, are the usual offenders.
Astroviruses, caliciviruses and reoviruses (rotaviruses) constitute the so-called "diarrhoea viruses". Such viruses often cause mini-epidemics in families, hospital wards, etc and are potentially very dangerous to seriously ill hospital patients. They contribute to the massive mortality caused by infantile diarrhoea in developing countries and are responsible for uncounted millions of deaths each year. One well documented source of infection by such viruses is the consumption of shellfish polluted by sewage. This means that in addition to health implications, they also have economic consequences for fishermen and the food industry.
An estimated 114,000 incidents of food poisonings are caused by bad seafood each year in the USA. In 2000, only 44 percent of seafood companies met the seafood safety rules called the "hazard analysis and critical control point system" (HACCP) issued in 1997 by the Federal Drug Authority (FDA).