Will We Be the Alien Space Invaders?

This is interesting because there is already some thinking that we may have contaminated Mars with the Viking probes from the 70s:

Do other star systems need protection from Earth life?

"The technology needed to send a robotic probe to another solar system is far in the future at best. But one scientist says it's not too soon to start thinking about how to avoid contaminating extrasolar planets with hitchhiking microbes from Earth."