I was on the Nieuw Amsterdam a few years ago, and there was an outbreak of the virus. My dinner table started with 8 people and ended up with 4.
We narrowed down the people that DID get sick at our table and what they had in common:
They were rarely outside.
They spent a lot of time in the casino (imagine the germs on tokens!)
We never saw them at the pool or beach.
HAL did a great job of trying to contain the virus - no handshaking, even at the Captain's party, no salt and pepper shakers on tables, servers served you portions at the buffet, all bedspreads were washed every 2 days, etc.
About 250 people got sick and the CDC was waiting for us at Port Everglades and cancelled the next cruise. I did not get sick at all.
It's hard to tell, when about 3,000 to 4,000 arrive by plane from all over the world in one day, where or who the virus might have come from.
The Norovirus has not crossed my mind since then, except to wash my hands more often.....everywhere.