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.
All of the guests came down with norovirus before they boarded the cruise. 
The statement says several of the guests had norovirus before boarding, so it's incorrect to say all of them had it. The word "all" may have confused the system. 
( z1 come-down :ARG1 ( z2 guest :mod ( z3 all ) ) :ARG2 ( z4 disease :name ( z5 name :op "Norovirus" ) ) :time ( z6 before :op ( z7 board :ARG0 z2 :ARG1 ( z8 cruise ) ) ) )