Amanda MacMillan is a health and science writer and editor. Her work appears across brands like Health, Prevention, SELF, O Magazine, Travel + Leisure, Time Out New York, and National Geographic's ...

In a viral video, a microbiolgist warns that Vitamin C supplements like Emergen-C and Airborne don't prevent colds.

Science says, if you're already coming down with something, it's too late—and it doesn't work well as a preventative, either.