Who made the decision to put the smallpox blanket on the rover? Is captain Bouquet in charge over at NASA? edit: In all seriousness though, this is a really interesting read. It's something I had not considered. The slightest contamination of the rover could spoil the virgin planet. This is the kind of article that the religious of the world could one day use as an argument that any life/microbes found there were originally brought there by us.
Where the ISS orbit is, unless they boost the poo out, it should all end up as meteor poo. But, I would think that there are some mechanisms by which biological material from Earth could get to Mars. Massive asteroid impacts might result in ejecting living material into space.
I would imagine there is a lot of space garbage out there floating around.... poo and all.