Preliminary tests identified the source of the outbreak as the highly contagious norovirus, which had struck several guests just before they boarded the cruise Nov. 3 in Rome, Carnival officials said.
Lots of people had norovirus in Rome.
The article says norovirus is highly contagious and that some people already had it when they got on the boat in Rome. They had to catch it somewhere, so logically some people in Rome had to have norovirus in order to spread it. There is no way to know how many people in Rome were sick, though. I think this confused the system because it can't work backwards logically.
( z1 have :ARG0 ( z2 person :quant ( z3 lot ) ) :ARG1 ( z4 disease :name ( z5 name :op "Norovirus" ) ) :location ( z6 city :name ( z7 name :op "Rome" ) ) )