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.
Norovirus isn't contagious at all. 
( z1 contagious :polarity - :domain ( z2 virus :name ( z3 name :op "Norovirus" ) ) :mod ( z4 at-all ) )