I'm with Zubrin on this. Every rocky planet in our system has been transferring material for billions of years via asteroid impact ejecta.
We have Martian rocks found on Earth, and we will absolutely find Earth rocks on Mars. There will be no Martian viruses because there is no macro fauna on Mars for them to parasitise. If microorganisms are on Mars they will be ancient and deep underground, just like our Earthling oxygen-phobic microbes are deep underground, and will probably be related to us.
It remains to be seen if there are any macro-fauna on the various ice worlds, and it's likely that microbe transfer between us and them is a hell of a lot lot rarer. That said, it is highly doubtful that humans shall be heading to the ice worlds within the next hundred years, so I'd not worry about it until it becomes a problem worth considering.
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brb welcome to my reed organ repair blog, Thu 23 Jul 2020, 22:04,
Reply)
What makes you think macro fauna are required?
Bacteriophages do just fine on Earth, I’m not sure why they (or something similar) couldn’t exist alongside simple organisms elsewhere.
(, Thu 23 Jul 2020, 23:52, Reply)