A variety of seafood-associated bacterial, viral, and parasitic agents cause illness in humans. These agents can generally be divided into three groups: (1) those that are known to cause disease in healthy adults; (2) those that usually do not cause disease in healthy adults but can cause illness in special population groups (children, immunosuppressed patients); and (3) those that are of uncertain pathogenicity for humans (Table 3-1). It should be kept in mind that these categories are not absolute and that agents could be moved from one category to another in response to new information and changing conditions. Pathogens such as Salmonella, V. parahaemolyticus, or Norwalk virus cause disease in normal, healthy hosts; however, the illness in normal hosts is generally mild and self-limited. In contrast, when persons who are immunocompromised or otherwise at high risk are infected, severe, life-threatening illness may occur (e.g., V. vulnificus).

Viral agents such as Norwalk and some bacterial agents such as Shigella require a very low infectious dose to cause illness (ca. 10² CFU for Shigella and theoretically one infectious viral particle for Norwalk). These agents are often transmitted by direct fecal-oral contamination and may pose a problem if they are allowed to contaminate food items after processing. Other bacteria generally require a higher infectious dose, which may be reached by time/temperature abuse of food products. Unfortunately, in the absence of volunteer studies the infectious doses for many of the viral and bacterial pathogens discussed are unknown; and even if such studies were carried out, it could not be proved that study conditions duplicate the wide range of human responses to natural exposure. However, it is probably accurate to say that the risk of infection increases with increased dose for most pathogens and that the dose depends to a large degree on the handling of the product after harvest, during processing, and in preparation.

The risk of exposure is dependent on a number of variables, including the type of seafood, the location, the water quality at harvest, and the way the product is handled after harvest. Risks are markedly different for each pathogen and each product, making it difficult to generalize about overall risk of exposure to infectious agents or to establish uniform water quality or product safety standards.

Infections with Vibrio species and other naturally occurring marine bacteria are generally associated with eating shellfish. Their numbers are dependent on salinity, temperature, and a variety of other factors, with temperature (>20°C) probably being the most important variable (Baross and Liston, 1970).

Of the enteric viruses, only hepatitis A virus, Norwalk virus, Snow Mountain agent, caliciviruses, astroviruses, NANB enteral hepatitis, and unspecified hepatitis have