Scientists have discussed this and agreed that we'd better not contaminate Mars with Earth life, at least not util we have confirmed if there is or isn't any indigenous life. This was formalized in article IX the Outer Space Treaty. Based on this agreement COSPAR was founded, and they defined the 5 categories of planetary protection.

Therefore, all vehicles going to Mars are thoroughly decontaminated before launch.

This requirement is a bit less strict for vehicles that are not intended to land, or going to other celestial bodies that are not deemed of any interest for the origins of life, or where Earth microbes are not expected to survive.