If we find alien life, can we avoid harming it?

As humans explore the solar system, the tantalizing possibility of discovering extraterrestrial life continues to pop up. But the goal of most scientists is to discover existing life on another world, not to accidentally bring it there from Earth. Whether travelers are robotic probes or human astronauts, scientists are increasingly faced with the challenge of preventing the contamination of alien environments. But if we uncover still-evolving alien life on another world, can we even justify going there?