You are correct that habitable is most commonly used with humans, but my post was clear I was referring to a microbe. Mars is also habitable by humans with reasonable extension of existng technology, but that is not what this thread is about. It is about the fundamenal question of whether life exists elsewhere.

As for the pathogenicity of Martian organisms answer my earlier question why this organism which could survive on Mars is not treated like the threat you think Martian organisms should be. The answer is in part because extremophiles have to be efficient in their metabolism to survive and be adapted to hash conditions. Conditions we find harsh are fine for them and the reverse is often true. For example an organism evolved to extract as much water as possible from a dry environment might find it' metabolism fouled by too much water in a normally wet environment. A real example of overdesign is D. radiodurans which while tremendously resiliant to radiation and capable of living in some environments no other organism isnot fond significanly in normal environments. It's extra DNA repair machinery is simply not metabolically efficent n normal environments and it gets beat out by organisms taking the reproductive shortcut of normal DNA copying. Containment is warranted simply because we have never encountered life from another planet not because we expect it to be hazardous. Our experience with extemophiles on Earth suggests they will not be hazardous.

I bet that if we find life on Mars it will be harmless from a disease standpoint. Once any Martian life is found to be harmless the issue will scare few.

Regards,
Mark