"Over the course of human history, scientists have identified only two instances of true virus superdodgers. That is, where a specific mutation in their genes makes people completely resistant to a virus. So that it slides off their cells, "like water sliding off a glass window," as Casanova puts it. In 2003, a team in London showed how some people never get a stomach bug, called norovirus, which causes vomiting and diarrhea. The researchers found that one mutation in their genes prevents them from making a molecule the virus needs to infect the cell. (In 1995, researchers in France figured out why some people appeared to never be infected with a species of malaria known as Plasmodium vivax. However, over the past decade, further studies have clarified that these superdodgers actually do become infected with the parasite; they simply don't show symptoms.)"