Aliens living among us unseen, that's total bullsh*t. If the aliens are so similar to us that they have DNA like humans, can eat same or similar food as us, then...well. Every human, in fact every multicellular organism is home to large amount of microorganisms. Some are harmful, some are harmless, some even essential, like E. Coli. Speaking about that one, it pretty much makes my point. What happens when E. Coli gets from your intestines to other organs? Yea, it is nasty. Very nasty. Now, imagine what would happen if alien microorganism gets released on Earth. No living creature on Earth would have antibodies against that...effect would be worse than in 15-18th century when explorers carried common european diseases to america and pacific islands...common, harmless diseases like flu depopulated entire islands there. If an alien disease would be released upon earth, it could cause massive ecological damage....
And since some of those buggers are symbiotic and essential, don't even start on stuff like "they have no diseases" etc...some things you just can't get rid of.
