When Jake Rosmarin, 30, boarded the MV Hondius, he gleefully posted on social media that the ship would be his home for 35 days as he traveled across the South Atlantic . Now, he is one of 18 Americans under observation at specialized health care facilities designed to treat people with dangerous infectious diseases after three people died and others were sickened by a hantavirus outbreak aboard the ship. He expects to spend 42 days at the National Quarantine Unit at the University of Nebraska Medical Center in Omaha , where 14 other American passengers from the ship are also being held. Another person who tested positive for the virus is in the Nebraska Biocontainment Unit, while two others are being monitored at Emory University Hospital in Atlanta, Georgia. Public health officials have said the risk of the virus spreading from passengers into the general public is very low and that healthy people are being quarantined as a precaution.…