I don't think the Hantavirus has been studied enough to have a test to determine whether people can build immunity to it. But I'd bet the Yosemite employees and also backcountry rangers get enough low exposure to it over a period of time that they develop an immunity.
On the other hand, a city-dweller who has never been in a Hanta-environment would have never been exposed, so would be way more susceptible.
