A real Martian invasion

The Mars Exploration Program Analysis Group (MEPAG) has advised NASA to beware of the risks of going to Mars. MEPAG reported that, "The most significant risk identified is that associated with the possibility of transporting a replicating life form to Earth, where it is found to have a negative effect on some aspect of the Earth's ecosystem." A beautifully worded way of saying the entire human race may end up dying in terrible agony of some horrible Martian microbe.