Life on Mars does not mean horror and doom. Extremophiles on Earth are not highly hazardous as a group and indeed few are hazardous at all. Any Martian organism would definitely classify as an extremophile. The reason extremophiles tend not to be hazardous is in part a simple matter of thermodynamics. Metabolic pathways use energy. When you spend energy to produce and other biochemicals needed to survive in extreme environments you are at a disadvantage in normal environments competing against organisms not wasting energy on unnecessary metabolism. The E coli in your gut could be one of your best defenses against Martian extremophiles.

There are other potential disadvantages such as excreting bases to deal with acid environments would lead to low pH in a normal environment. Pathways designed to deal with a pH extreme would most likely be screwed up at normal pH or the opposite extreme. Proteins and DNA evolved to work in hot environments becomes too rigid in normal environments and cold adapted biopolymers can become too flexible. Another key factor is that most extremophiles are autotrophic meaning they produce their biochemicals and energy from inorganic chemcals not by attack on other organisms. This is another simple outcome of evolution - there are few potential prey species in their environments.

You don't have to take my word on this stuff either medical and biological experts have come to pretty much the same conclusion and the reports are available online.

Life on Mars could delay human missions by increasing the number of precursor robotic missions to evaluate it's potential hazards,. However, given the almost inevitable green light I think life on Mars will become a powerfull driver and increase the probability of eventual human exploration due to our fascination with the possibility of extraterrestrial life. I don't fear it in the least as a proponant of human colonization of Mars.

Regards,
Mark