Shellfish are especially "friendly" to viruses because shellfish pump water through their bodies and thus, concentrate food and contaminants, such as viruses, from the water. Once the virus is on or in the shellfish - a live host - it may persist for a longer time than if it were suspended in the water. Some shellfish are eaten raw or lightly cooked, which increases the risk of foodborne illness.
Produce used for salads, lettuce, spinach, etc., grow low to the ground where they are more likely to come in contact with contaminated, organic fertilizers, such as manure. Also, sometimes produce is irrigated with contaminated waters or picked by farm workers with poor hygiene practices. The complex, multi-layered surfaces of salad produce are more difficult to clean after picking than, for example, the surface of an apple or potato. Finally, because salads are usually eaten raw, there is no heating step that would inactivate the viruses
Source: Excerpted from FDA/CFSAN Food Safety A to Z Reference Guide, September 2001