“the epidemic RAGED (emphasis my own) on the ship”
Seriously? Hyperbole much?
The Diamond Princess was a Petri dish of infected people with a very large (in proportion) of the elderly on board. When you have infected workers cooking, cleaning rooms, and large groups of individuals socializing, you greatly increase the chances of transmitting it to the others on board.
Ever heard of Typhoid Mary? Some municipalities have been forced to actually arrest those that have been infected by very infectious diseases to keep them from infecting others. The infected person adamantly refused to self-isolate.