Health officials in several countries are trying to identify and follow people who may have been exposed to hantavirus on a cruise ship. In addition to contact tracing, they are trying to learn more about the virus as fast as they can. Questions include whether it has mutated and how exactly it spreads. Hantaviruses usually spread when people inhale contaminated residue of rodent droppings. While human cases are rare, small outbreaks have been documented around the world. And the Andes virus implicated in the outbreak unfolding on a cruise ship now in the Atlantic Ocean may be able to spread between people in rare cases.