Why would there be viruses on a planet without any biological hosts????
This worry does seem a little silly, doesn't it? Viruses evolved here on earth. An extraterrestrial organism have RNA similar to ours is rather farfetched.
Maybe they are waiting for the rocks to turn into people. According to one theory I've heard, that has happened and probably will again. /sarc
Some scientists have theorized that there were viruses on the Earth before there were biologicals.
Maybe some Soviet rocket tech coughed on USSR Mars probes “for good luck”?
(Not that I know much about viruses, but I did hear that Hep C can survive for 6 months in a syringe of blood at room temp. And NASA reactivated Staph bacteria from parts of a NASA Moon probe that an Apollo mission brought back. It was up there for at least a year prior to the Apollo landing.)