The reported illnesses are associated with the naturally occurring bacterium V. parahaemolyticus, which is most prevalent in the summer when water temperatures rise, according to the health department.
Since mid-August, the health department has received three confirmed reports of the illness and three additional unconfirmed cases, said Kristy Michie, the department's supervising public health epidemiologist.
While the majority of illnesses caused by the bacteria are not life-threatening, symptoms can be extremely unpleasant and usually include diarrhea, abdominal cramping, nausea, vomiting, fever and chills.
In people with compromised immune systems, the illness can lead to more serious medical conditions that can quickly become life-threatening, according to the health department.
Michie said more people may have become sickened by the bacteria, but only experienced mild symptoms and didn't report it to the health department.
Because the bacteria is naturally occurring and doesn't imply sewage contamination, eating raw oysters from so-called clean waters or from restaurants won't protect people from becoming ill.
Health officials are advising people to avoid all oysters that are served lightly steamed, marinated or prepared as Rockefeller.
Eating raw or undercooked oysters with hot sauce or while consuming alcohol also does not kill the bacteria or protect people from becoming sick from it. Only heat can kill the bacteria, according to health officials.