The Enchantment and the Mariner were both affected by the virus. The virus can spread between ships in a number of ways.
The comedians, singers, entertainers will fly from port to port to work different ships during a week. They can carry the virus between ships.
Food borne viruses can come from the same suppliers but distributed to different ships. A contaminated shipment of, say, shrimp, can be distributed amongst different ships.
Staff members will travel from ship to ship to back fill for co-workers taking vacations or other leave.
Dock workers handling the luggage can transfer the virus from luggage to luggage. They will typically work all cruiselines in a port and they can pick it up from loading up a RCI ship and then moving across the harbor and pass it onto a HAL ship when loading it.
A co-worker of mine got sick on the Voyager last year and the doctor told him the only way to get over it was to starve himself for 48 hours. No food. Well he couldn't bring himself to do that for the first day and then he abstained for day 2 and 3. After that he was fine.