Realistically, anything Earth-like would likely be uninhabitable. Aside from the forward and backward contamination issues, an entirely alien biosphere wouldn't contain any diseases or poisons adapted to us, but would be saturated with things that are moderately to severely toxic or just noxious, and complex organic substances that our immune systems have never encountered before, some of which would be likely to cause severe allergic reactions.

In short, it'd probably stink horribly and send you into anaphylactic shock, and if it didn't, it'd probably have environmental toxins that would kill you slowly. Habitable environments are those with the natural resources needed to support habitats where we can support Earth life, not those already filled with alien life.

Mars? Oh no, perchlorates! Yeah, about 0.5-1% of the regolith consists of salts twice as toxic as table salt that are unstable, easily washed out with water or decomposed by heat or reducing agents, and which do not bioaccumulate. Worry more about heavy metals and long-lived organic compounds leaching out of plastics and such.