Mars meteorites have been found on Earth, and there likely are Earth meteorites that have crashed on Mars. If you take any random piece of rock from the surface of Earth, the odds are there will be some microbes on it. Some microbes from Earth can survive as they travel through space (see the Surveyor III part returned by Apollo).
Could unicellular life forms end up on Mars after being transported by an Earth meteoroid? Would it be possible for some to survive the trip to Mars? What about through the martian atmosphere?