Are Extraterrestrials a Threat to Mankind?

Description

What would be the impact for Earth if or when humankind encounters extraterrestrial life? Whether that alien life is microscopic or sentient, discovered through human exploration or by an alien communication or through the sudden appearance of spaceships above out world, would Earth and humanity be in danger? This title explores opinions about the possible effects of alien contact held by those scientists and thinkers who are actively involved in the search for extraterrestrial life or are seriously considering how Earth should react if contact is ever made.