Aliens, that is life that is from a place that isn't Earth, has been a hot debate for centuries. Most people in today's world accept it as fact that we aren't alone in the universe. There are are organizations like SETI that devote all of there time monitoring the cosmos for any kind of proof that life (especially intelligent) exists.
Why would monitoring the skies tell us whether or not there are others out there? We have been beaming radio and television broadcasts out into space since the invention of the radio and T.V. It stands to reason that an alien race would be doing the same. They don't have to be sending us a message per se, we could just simply eavesdrop on there particular type of entertainment.
The question is, is this a good idea, and could it be dangerous? Yes it is a good idea. It would answer a question that mankind has asked for hundreds of years. Having proof that there is intelligent life outside of Earth would be bigger than when man stepped on the moon.
Is it dangerous? The answer to that one is yes as well. Why is it dangerous? As far as a space faring species goes, we are barely infants. We have yet to send a human farther than the moon. This like a little kid that has finally made it across the street. It doesn't mean that he knows the neighborhood.
We don't have any idea what kind of life forms are out there. Alien life forms could come in any shape or size. They could be friendly or extremely dangerous. While we are listening for some kind of a signal from the cosmos, we are bombarding space with all of our television and radio programs. If you think about it, do you really want first contact to be based on a movie that was made by Hollywood? That is a scary thought in itself.
Let's assume for arguments sake that we do pick up a signal that is from an intelligent race of aliens from another system's planet. We take the next step and communicate via radio waves or some other type of technology. Eventually we make contact.
Now we are to the point of actually meeting an alien life form. To make it simple, we will say that they are friendly (although there is no guarantee of that). They could very possibly teach us new things, after all they have the ability to travel to our planet when we can't make it past our own moon.
Now we have to go back in history. What happened to the Native Americans when the Vikings landed in the “New World” and later Columbus and the English settlers? The indigenous populations were wiped out by common germs, bacteria, and viruses. There is no reason to think that a being from another planet wouldn't have the same effect on the human race. The same could be said for the aliens. With different immune systems, even entire metabolic processes a minor infection to either species could be deadly to the other.
So, even if the aliens had the best intentions, even being in their presence would be the same as being at ground zero during a biological warfare attack.