I´t could be really dangerous, couse if life on earth was introduced by a comet maybe some microbes ended up on other parts of our solar system and are well compatible with life on earth, and then if those microbes are stronger maybe it will kill the earth microbes (as I said earlier)
This is one of the difficulties aliens would have on earth,(among us,theories) I think, Bakterias and such, you know how sensitive life is to organism in, for example, water,air and food. ( been in a foreign country?? then you know what I mean )
(just writing when it pops up, does it make sense?)