Dr. Stephen Kornfeld, an American physician aboard the MV Hondius cruise ship, has been treating patients during a hantavirus outbreak that killed at least three people and infected at least five confirmed cases. Health authorities in a dozen countries are now tracking passengers who dispersed worldwide after the ship docked, and Kornfeld told CNN that quarantine protocols remain under active revision as the situation develops. The outbreak represents a rare case of a major infectious disease cluster on an international vessel, triggering coordinated international response efforts coordinated by the WHO.