You may remember the Hoyle disciple (Barry DiGregorio) that posted here screaming against a mars sample return because the viruses might kill us... he also advanced the Hoyle garbage about how the Great Flu was an interstellar virus cloud that fell to earth.

Its nonsense.
Viruses are simple, and BECAUSE they are simple they are very specific.

Viruses and their hosts co-evolve and become quite specific to each-other. The more evolutionary distance between two species the fewer viruses they share.

When did you last hear of someone calling in sick because they caught that nasty strain of the Cauliflower Mosaic Virus going around?

We share many viral pathogens with chimps, fewer with dogs, none that I could find with fish and insects, and none with plants.

More complex bacteria can be more general, but viruses are not.

Even IF an alien ecosystem developed with something like DNA and RNA (Highly unlikely) and even if it had the same identical building blocks of DNA and RNA (ACTG for DNA, AUCG for RNA)- astronomically unlikely, and even if it was operating with the same 'language' and enzymes and proteins...(Damn near impossible)... even granting all that, a cucumber would be far more related and similar to us than anything that virus was evolved to handle.

It just wouldn't be compatible.

Its like a Vulcan and a Human engaging in sex (even assuming their naughty bits are compatible)- for any progeny to result wouldn't just be surprising, it would be fuckin' Spocking!

More complex and independent bacteria like microbes might be a threat... if they have the machinery that allows then to thrive with nothing more than a source of raw material... (assuming its not expecting unique easy to digest proteins that it has evolved specifically to handle, and that our alien molecules are not immediately toxic to it) But how likely is it that something like that can survive extended exposure to cosmic radiation as it travels between the stars?