A sudden, fast-spreading norovirus outbreak that sickened almost half the residents at a Victoria, B.C., seniors’ home and may have led to the deaths of nine of them appears to be waning.

And now, authorities are trying to figure out how it spread so quickly and sickened so many.

A total of 106 of the 200 residents at the Selkirk Place nursing home have become sick, along with 53 staff members. As of Wednesday, only 11 residents were still showing symptoms, chief medical health officer for the Vancouver Island Health Authority Dr. Richard Stanwick said.