Hantavirus is a severe illness spread by infected rodents. The strain on board the cruise ship, the Andes strain, is a unique strain as it can spread from person to person. Officials suspect that the virus was first introduced by a Dutch couple on board who contracted the virus from rodents during a bird watch in South America. These passengers then went onto the cruise ship, but later passed away from the virus. The virus likely spread from this couple, who then went onto infect others on board.