What is hantavirus, the illness suspected to have killed several people on an Atlantic cruise ship?

A Dutch cruise ship with nearly 150 people aboard, including 17 Americans, was waiting for help off the coast of Cape Verde in the Atlantic Ocean on Monday after a suspected outbreak of the rare hantavirus killed three passengers and left at least three others seriously ill, the World Health Organization and the ship's operator said. Here's a look at what we know about the virus.