I think you are being a bit too optimistic when it comes to close contact with aliens. There would be probably years worth of research on whether such contact would be safe done from both sides as any space-faring civilization would very well know of the risks of viruses, bacteria, and similar things.

Also, a 3D printer capable of printing viruses would have to be almost impossibly intricate as it would have to be capable of literally printing DNA. And their attempt could easily fail because they would have no prior knowledge of our immune systems so it could easily just defeat it. It would be a fifty/fifty shot at either it does nothing or it wipes us out and they would have no knowledge on whether it worked because of the time delay.