Norovirus. As the Raleigh News & Observer reports:
Noroviruses are more contagious than most viruses. People shed millions of the virus particles in their stool and vomit, Moore said, but it only takes between 10 and 100 particles to infect someone.
“It’s such a common infection because it’s very hardy. It can live on surfaces in the environment if it gets onto them, and the usual types of disinfectants aren’t effective against it,” Moore said. “If there are areas that have been contaminated, you have to use a dilute bleach solution to kill the norovirus.”
And there’s a new strain of norovirus out this year, so most people don’t have immunity. Oh, and alcohol-based hand sanitizers aren’t effective against it.