Are Martian Microbes Dangerous?

According to an article in the London Times, they might be:

EARTH must take precautions to avoid contamination from lifeforms that must now be presumed to exist on Mars, leading scientists gave warning yesterday.

Potentially deadly microorganisms could be returned to Earth on a probe which is being planned to collect samples from the Martian surface.

………

The risks are twofold: probes sent from Earth may contaminate Mars with terrestrial bacteria, wrecking future studies of Martian life; or, more important, bacteria brought back from Mars may contaminate the Earth with unpredictable effects.

I doubt if Mars bugs are that dangerous. For one thing, as the same article mentioned:

• A meteorite from Mars found in the Antarctic seemed to contain structures suggestive of life and reignited the possibility that Mars had once been home to life in a microscopic form

We've probably been exposed to Martian microbes already.