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.