Many shellfish are filter-feeders, so they strain microbes from sea water throughout their lifetimes. This means that over time, they can accumulate high concentrations of pathogens that were present in their environment. In particular, filter-feeder (especially oysters) have been found to carry a disease agent called Vibrio. Vibrio infection can result in diarrhea, septicemia, and wound infections. Cases are rare but increased 75 percent from 2006-2008 to 2013.
"As a food microbiologist, I'd like to eat raw oysters – I grew up eating them – but I don't eat them anymore," says McLandsborough. It's hypothesized that the jump in vibrio infections may merely reflect better monitoring but warming seas are another potential cause.