I wouldn't say that Zubrin's "Dutch Elm" analogy is faulty; his point isn't that parasites can't cross the divide between species, but that they can't cross a *wide* divide between species. parasites jump between humans and apes quite freely, because we're very nearly the same species. They jump between us and other mammals with less frequency, between us and reptiles less frequently than that, and between us and plants -- never. Anything living on Mars will be a LOT less related to us than terrestrial plants! It probably won't even be kin with our conventional prokaryotes (maybe the archaea).

Life on Earth is CLEVER. If there was any concievable way for parasites to figure out how to cross such wide species gaps, the way B.D. likes to hallucinate, then it certainly would have done so already. There is no reason to suppose that the evolutionary path on Mars -- which, if it even has a biosphere, is far more static than our own -- could come up with such an ability.

Cheers,

Nathan.