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Our Man In Black
bot writes "A recent Slate article covers the onerous responsibilites of the Planetary Protection Officer. He is tasked with preventing contamination of earth by alien organisms, and 'forward contamination' (contamination of other planets with earth germs). There is also a published protocol (PDF link) for avoiding Martian bugs."
I don't think you quite get the jist of his job. Say a probe lands on Mars and brings back some rock samples to Earth. Unfortunately, the probe stepped in something nasty, and brought back Mars Anthrax as well.
In addition, some benign fungus got attached to the probe just before we packed it up and sent it to mars. Unfortunately, the fungus is not so benign to the Martian plant life and wiped out entire ecosystems there.
Sadly, at the rate we are preparing to actually send people to Mars and bringing them back, I am betting this won't be a problem for the current holder of the officer, or his next eight successors.
I Am My Own Worst Enemy
The chance of an alien microbe getting a toehold on Earth is pretty much zero. Most every niche is filled by a lifeform that has four billion years worth of ancestors that didn't die before they breed.
If a Martian microbe shows up, it gets eaten in 30 seconds by some terrestrial super-optimised (for earth) bug. The martian bug's super radiation protection, cold-protection, etc, just means it has misallocated resources for the terrestrial environment.
Notice that people get infected by bugs that have evolved attacking animals similar to people (e.g. primates, mammals, some birds.) The nasty ones come from animals similar to us. We have little to fear from reptile bug, less from plant bugs, and nothing to fear from things that attack fungi. Martian bugs would be like tourists from Iowa trying to infect New York City.