Island Health says norovirus is likely to blame after more than 100 people who ate raw oysters in Tofino earlier this month fell ill.
Roughly 120 people, many of whom had attended the Clayoquot Oyster Festival, suffered gastro-intestinal symptoms last week.
But Island Health says people got sick at more than one location, and that people reported being ill over the course of several days.
They say it appears everyone who became ill consumed raw oysters from the same supplier, who is not being named.
"The predominant amount of evidence clearly shows that raw oysters at that particular point in time that were available were the cause of the illness," said Paul Hasselback, a medical health officer for Island Health.
Hasselback says they are now investigating how the affected oysters were harvested and transported.
"It's unfortunate when these incidents do occur. We now need to figure out what occurred so that we can learn from that and make sure it doesn't happen again."
Hasselback says everyone who fell ill has since recovered.
With files from Megan Thomas