Most human cases of bird flu in North America have been mild, a fact that’s underscored by a new study of the first 46 confirmed human H5N1 infections in the United States this year. But the case of an ill Canadian teen stands out because of its severity and because the source of exposure remains a mystery.

Dr. Craig Shapiro, an infectious disease specialist at the Children's National Hospital, joined DC News Now to share norovirus symptoms people should look out for as cases continue to surge.

The federal Centers for Disease Control and Prevention reported double the cases of norovirus compared with previous years in early December after outbreaks were reported across the country.