The state Department of Health and Hospitals is investigating additional reports from people who say they got sick after having eaten raw oysters.

DHH spokeswoman Olivia Watkins said department staff are looking into calls received Monday about possible illnesses, but there's no definitive answer yet on whether those illnesses are a result of eating raw oysters.

Tuesday's announcement is the continuation of an investigation started last month that resulted in the precautionary, temporary closure of three oyster-harvesting areas.

Norovirus causes stomach flu-type symptoms that can last for a day or two and include nausea, vomiting, diarrhea and stomach cramping, according to DHH.

"We know the virus comes from humans," Dr. Jimmy Guidry, state health officer, told the Louisiana Oyster Task Force on Tuesday. "The question is how does it get there (in the oysters)."

Water in three areas of the Louisiana coast have tested negative for fecal coliform, but that doesn't mean it wasn't there at some point, he said.

"The fecal coliform could be gone, but the virus will still be there," Guidry said.

Rainy weather this fall and winter could have pushed more water into certain oyster beds and brought with it fecal coliform from sewers and septic systems, he said.

In addition, it's unclear whether the oysters became infected with norovirus in the water or in the post-harvest handling and preparation for consumption, he said.

The virus is passed from person to person with exposure to fecal matter from someone who is ill, although the virus can be killed in oysters by fully cooking them, he said.