What counts as 'close contact'? Why the risk of hantavirus transmission is tricky to define

Photo taken from outside a charter bus. two individuals wearing respirators and plastic gowns over their clothes sit inside, as one waves out the bus window.

In the aftermath of the hantavirus outbreak on the cruise ship MV Hondius, many of the 150 passengers are currently being monitored or undergoing up to six weeks of quarantine, based on the virus's lengthy incubation period. Public health authorities have emphasized that person-to-person transmission of Andes virus — the only type of hantavirus known to spread between humans — is rare and associated with "close and prolonged contact."

But there's no clear and consistent definition of what that means.