I know that without water life would have never begun, but I still don't fully understand why it is necessary. It is still big discovery though, if life is that common I think we should be more careful during migration there, "Marsian" common cold could wipe out the entire human colony.
Of course martian life could be different than terran life, and that would be interesting too. But there are reasons to suspect that liquid chemical reactions was necessary at the genesis of life, and water is a uniquely optimal liquid for this purpose.
A martian virus is unlikely to do anything to your immune system. For one thing it is unlikely that the virus would even use the same genetic codes (A C T G) as terran DNA, unless we share a common ancestor, and even in that case it wouldn't do anything against you & your immune system would eventually create an antibody and wipe it out. We fear things like bird flu and swine flu because they develop in close proximity to humans in analogous systems, honing attack vectors specifically targeting some weakness of the animal body. An alien virus would likewise be good at attacking aliens, not us.
