Seems to me you're asking two questions or people are giving two answers.
Knowing that there exists sentient beings, with whom we might be able to communicate, living on another planet in our galaxy would be somewhat
thrilling, and I think it's likely.
On the other hand, knowing that there were actually aliens here on Earth, now, would fill me with fear and dread.
But the reason would be fear of their infectivity, not of the aliens, themselves.
Though I presume they'd take some kind of precautions, I think it's nearly impossible to guard against accidental contamination with an alien plague
were extraterrestrial beings actually to come to Earth in a physical form.
Such a plague would be likely to be so different than anything our immune systems had seen that it would be immediately devastating to all humans, I
fear.
In fact, this is one way I know that we don't have any aliens here now, and we probably didn't in the recent past - the lack of any evidence alien
virus or microorganisms. IOW, we're still here.
(Of course it's possible that we have had such a thing before recorded history, hard to say).
Yep, in my opinion, H.G.Wells had it right.
Perhaps, in the distant future we will make a visit to other civilizations, but to be safe, we'll do it in a virtual or holographic form. (it would
also save gas.
)
[edit on 9-10-2007 by Badge01]