There was an outbreak of Norwalk here in Victoria last winter. It's quite nasty. My ex was working in an extended care facility, and in spite of careful hygenic practices, all 4 of us eventually came down with this virus, which knocked each of us out of commission for between 2 and 10 days.
Several of the residents in the nursing home died from it.
As for it being a terrorist attack, I sincerely doubt it: when you're sick with this, you are pretty near completely incapacitated. Just getting up to go to the can is a struggle, and any walk of more than about 50 feet is going to mess your underwear
This happens a lot. And not just with Norwalk. The worst place for a sick person is in a hospital if the hospital's infection control procedures aren't up to snuff. Same for a senior in an elder care facility.