As Zubrin says, they’ve been deep underground on Mars for billions of years. They haven’t evolved to infect humans, or any animals living on Earth, and wouldn’t be a problem for us.
This is similar to bacteria deep inside Earth that wouldn’t know what to do with us if it encountered us; it wouldn’t say, “Oh, let’s go infect that guy!” They wouldn’t know how to do that even if the thought occurred to them. We’ve lived “side-by-side” on Earth with them for billions of years (geological processes do bring them to the surface once in a while) and never has their DNA turned up in a sick person, which might indicate infection.
A pathogenic species and “victim” species have to evolve together, continuously, for a long time in order for the pathogen to be able to infect the victim. Well, at least, that’s what I get out of listening to Zubrin in this video.
As Zubrin says, they’ve been deep underground on Mars for billions of years. They haven’t evolved to infect humans, or any animals living on Earth, and wouldn’t be a problem for us.
If he really said that, he's just displaying scientific ignorance -- one has nothing to do with the other.